Okay, that took a while, but here is a list of ALL the Gr.1 cars. Top speeds listed, brief description of what I thought about them written, and if you're feeling the "too long, didn't read" itch, I left a ranking at the end of it all. Hope it's helpful 

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(ALL CARS TESTED WITH TCS 0, BOP ON)
Alpine VGT Gr.1:
-No slipstream: 357km/h
-Slipstream: 358km/h
Notes: Very easy to drive and balanced overall, even with TCS 0 you can really have a lot of freedom with the gas pedal. Good all-rounder for the ones that aren't used to Gr. 1.
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Audi R18 '16:
-No slipstream: 334km/h
-Slipstream: 341km/h
Notes: Compared to the Group C cars, I was a sitting duck even at top speed. The R18 also is pretty nervous out of corners if you stomp on the gas because it's not AWD like the Porsche 919 for example, and in anything resembling an acceleration zone, I was left in the dust. Doesn't turn too well either...
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Audi R18 '11:
-No slipstream: 350km/h
-Slipstream: 357km/h
Notes: If I told you this was better around La Sarthe as a whole than it's newer model, you probably wouldn't believe me; but it is. So... that was a shock. The 2011 R18 is surprisingly simple to drive, no AWD, no hybrid power to worry about here. It is just here for a race. And it's not a sitting duck against the Gr. C's either. Comparable to the Alpine but it has a lower top speed if not in the slipstream.
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Audi VGT Gr.1:
-No slipstream: 348km/h
-Slipstream: 358km/h
Notes: First of all, this car sounds like a pack of ANGRY RHINOS! UGH! If Audi ever made this car in real life into an actual prototype racer, I'd go to Le Mans in a heartbeat just to hear it whizz past! Anyway, in game it's kind of like a jack of all trades. Not as good as the hybrids in the acceleration department, but it has their cornering capability, while also not being as good as other cars in top speed, but it certainly is better than most, especially in the slipstream. Another good, well-rounded car. Although you won't need 7th gear, not even in the slipstream; the gear ratios are messed up and 7th is like an overdrive gear.
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Bugatti VGT:
-No slipstream: 348km/h
-Slipstream: 351km/h
Notes: Honestly, expected much better from the walking piece of W16 art. The Bugatti doesn't have the top speed to rival most of the field, it's not that much better than others in the cornering department; showing a lot of noticeable understeer, and it also can get easily upset while braking through the Indianapolis corner. Overall, wouldn't recommend unless you like a challenge or you're an alien smurfing in an E-B account...
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SRT Tomahawk VGT Gr.1:
-No slipstream: 352km/h
-Slipstream: 354km/h
Notes: The SRT, sorry to say, is quite unremarkable in terms of pace. The top speed is fine, and the cornering is okay, but that's really the key word for the car; just "okay". It gets out-accelerated by almost anything from what I've seen and the brakes are noticeably worse than others (I had to start braking a bit before the cone markers every time, most noticeable ones were at the Mulsanne corner and the braking zone before the Ford chicanes).
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Jaguar XJR-9:
-No slipstream: 366km/h
-Slipstream: 369km/h (nice)
Notes: The Jaaaaaag is almost exactly comparable to the 787B; both are consistent, reliable cars with high top speeds that won't tend to kill you as much as other Group C's. The XJR-9 was a bit more nervous in 2nd gear than the Mazda but not very noticeable, although I feel the 787B has the edge due to it's better (but a bit negligible) top speeds across the board.
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Hyundai N 2025 VGT Gr.1:
-No slipstream: 352km/h
-Slipstream: 354km/h
Notes: Pretty much like the SRT, the Hyundai is unremarkable, but it is marginally better than the Tomahawk. The N 2025 shares many of it's attributes with the added bonuses of not getting out accelerated by almost anything and also a full-time AWD system, which gives you more confidence when stomping the gas. Other than that, I'd sadly stay away from it too, because it looks quite cool. Too bad the engine sounds like a rotary that's been mutilated and thrown into a blender.
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Mazda LM55 VGT Gr.1:
-No slipstream: 360km/h
-Slipstream: 361km/h
Notes: The LM55 joins the ranks of the all-rounders quite comfortably. It's full-time AWD system gives it amazing stability under acceleration, it has a stellar top speed, able to hang with the non-hybrid modern prototypes like the 908 HDi (Group C's still outrun it, though), and also slightly out-edges the Alpine VGT in acceleration. Cornering is okay, does tend to understeer if you push it, but that's the price to pay for a car that is fast and is easy to drive. The Mazda also has the gift of rotary... ah, great sound...
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Mazda 787B:
-No slipstream: 365km/h
-Slipstream: 371km/h
Notes: If the LM55 was good, then here we gave it a bottle of Rich Energy and watched as it mutated onto an out of control, beast of a prototype. The 787B is as easy to drive as a Group C can get: surely you can spin it out, but it's gonna take some effort to do so. It gets out-accelerated by almost every hybrid, especially the TS050, but then it goes...
"NOT WHEN I GO INTO MAAXIMUM OVEEERDRIIIIVE!" *shifts into 4th gear*
"GAAAAAS GAAAAS GAAAAS! I'M GONNA STEP ON THE GAAAAS!"
...and it goes past the TS050 like it's James May's cheese. That is, if you have the guts to hang with it on the curvy roads, as the 787B is not nearly as cutting edge anymore as it was in the 90s. Still, it'll probably win races with it's consistent performance, and put a smile on your face too.
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McLaren VGT Gr.1:
-No slipstream: 354km/h
-Slipstream: 360km/h
Notes: This one actually made me a bit sad, since it's got a winning formula that is ruined by one small and yet important thing. The McLaren VGT corners great, brakes decently well and doesn't have bad gear ratios for a hybrid, with it's opportunity to get up to such a high top speed and all (pretty much on point with the all-rounded cars like the Alpine VGT and the LM55).
However, the 'Laren's downfall is indeed the battery and how it uses it. First of all, the battery is smaller than the one they use in the ****ing G-Wiz, because it runs out in like, 2 seconds. Secondly, the car doesn't use it smartly, like the 919 for example; it goes from nothing in 1st gear, nothing in 2nd gear, nothing in 3rd gear, to WOAAAAAH POWER SUUUURGE in 4th gear that lasts for 2 seconds and doesn't do much in the long run. And because you're in 4th gear and up a lot in La Sarthe, the only good-ish opportunities to charge the battery are the Mulsanne corner and the Ford chicanes, and then you'll just use it up in no time...
The good thing about it though is that it doesn't get that nasty wheel lock-up like the 919 does when you full throttle it in a low gear with the battery fully charged. You know what I'm talking about. Overall, I actually like the McLaren a lot, shame that the technicalities make it worse than it should be.
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Sauber Mercedes C9:
-No slipstream: 357km/h
-Slipstream: 369km/h (nice)
Notes: Honestly, not much to say about the C9, other than it's weaker than the 787B and the Jaaaaaag in my opinion. It tends to lose traction more easily, and has lower top speeds (especially without the draft). It's not that much worse though, and I felt it was sliiightly better in the Porsche / Corvette curves, but it could've been placebo.
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Nissan GT-R LM NISMO:
-No slipstream: 334km/h
-Slipstream: 347km/h
Notes: Uh... want to lose DR? Drive this car.
...what? What else do you want me to say? Okay, I'll say some things about it. First of all, whoever thought making it FF was a good idea, props to you because it was the worst idea ever. The tires were already slightly worn by the end of lap 1, unlike pretty much every other car (even the AWDs and the Group C's). Secondly, this is the only prototype aside from the Group C's to have a 5-speed gearbox... yeah. Thirdly, the battery doesn't even charge all the way (seriously, try it for yourself) and it only charges while braking (decelerating doesn't count). And a fun fact, it pulls to the left on it's own when the tires are worn. Try it.
...can we go back to an actual LMP car please? Thanks?
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Nissan R92CP:
-No slipstream: 371km/h
-Slipstream: 373km/h
Notes: The R92 combines pretty much all the good things about the Group C cars, and gives it some polishing. It's as easy to handle at low speeds as the Jaguar, while being Mazda-fast and some more on the straights, and it eats pretty much any car that isn't the 919 / TS050 for breakfast in acceleration. I think this is one of the best of the Group C's, but it has noticeable understeer tendencies at the Porsche curves.
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Peugeot L750R HYbrid VGT:
-No slipstream: 336km/h
-Slipstream: 341km/h
Notes: Hey, GT-R LM! You've got mail!
*To: Nissan GT-R LM NISMO
*From: Peugeot L750R HYbrid VGT
*Subject: Friend Request
hey, wanna be friends?
The L750R is as unremarkable as one can get. Battery power is unnoticeable and might as well not be there, pathetic top speed and okay acceleration. The things it has going for it is cornering and braking; rather good, but pales in comparison to the disadvantages it has.
...at least it's right wheel drive...
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Peugeot 908 HDi:
-No slipstream: 351km/h
-Slipstream: 352km/h
Notes: Compared to the L750R, the 908 isn't much better. Trades the braking capability for a better top speed and it does have a bit more wheelspin, especially in lower gears. However, I would consider this a step up from the L750, but it's still more toward the bottom of the spectrum.
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Porsche 962C:
-No slipstream: 363km/h
-Slipstream: 373km/h
Notes: The 962C is like the R92CP, but without the understeering tendencies at high speed. However, what it gains in cornering, it loses in grunt, as it gets out-accelerated by Nissan's Group C offering. Shockingly so, in fact I was almost asking myself if the secondary turbine of the 962 had stopped working (yes I like my Initial D references). Overall, it might be a hard choice between the two.
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Porsche 919 Hybrid:
-No slipstream: 338km/h
-Slipstream: 338km/h
Notes: Porsche's modern LMP offering is wildly popular in almost every Gr.1 daily race, but it doesn't look like today will be the day for it to shine. It's top speed makes it a sitting duck, but it has the cornering capability and the monstrous acceleration to bring the fight to any other Gr.1 car in-between the Mulsanne corner all the way to the Tertre Rouge corner. After that, though... eeeesh.
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Toyota TS030 Hybrid:
-No slipstream: 350km/h
-Slipstream: 353km/h
Notes: Toyota's older hybrid is akin to the 919; good acceleration, great cornering and nice braking capability. However, just in my opinion, the TS030 edges out the 919 simply because of the higher top speed it possesses, which makes it not as defenseless as one might expect.
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Toyota TS050 Hybrid:
-No slipstream: 344km/h
-Slipstream: 353km/h
Notes: However, the TS050 puts those qualities in what is, in my opinion, an even better overall package. I found the TS050 only added to the previous model's strengths, without giving it weaknesses (apart from the slightly inferior top speed without slisptream asistance). The TS050 is really up there, but will it be able to offset the Group C's? In my opinion, no, but we'll have to see.
Therefore, we conclude the list. And here's my rankings:
-1: Porsche 962C
-2: Nissan R92CP
-3: Mazda 787B
-4: Jaguar XJR-9
-5: Sauber Mercedes C9
-6: Toyota TS050 Hybrid
-7: Toyota TS030 Hybrid
-8: Porsche 919 Hybrid
-9: McLaren VGT Gr.1
-10: Mazda LM55 VGT Gr.1
-11: Alpine VGT Gr.1
-12: Audi VGT Gr.1
-13: Peugeot 908 HDi
-14: Hyundai N 2025 VGT Gr.1
-15: Audi R18 '11
-16: SRT Tomahawk VGT Gr.1
-17: Audi R18 '16
-18: Bugatti VGT Gr.1
-19: Peugeot L750R HYbrid VGT
-20: Nissan GT-R LM NISMO