I used both the CLK-LM and the F1 GTR for these events, the CLK-LM detuned to the low 400s and about 200kg of ballast and the F1 GTR detuned to the low 500s with racing hards and slight ballast, the CLK-LM was slower due to having less power but both were a breeze in all of the 800pp races and the GT WTC. The F1 GTR is very good on fuel only having to pit once on lap 10-11 on Fuel setting 4 and twice if using fuel setting 1 on lap 7->8 and again on lap 14->15. The CLK-LM was pretty bad on fuel having to pit atleast 2 times every 6 laps or so but still better than the Gr.3 cars. The F1 GTR is my new grind car to use until I reach my goal to where I can afford both another CLK-LM and F1 GTR, i'm already very close to getting enough for the LM as the race pays $727,500 every 26~mins which means you can net an approximate $1,455,000 per hour if doing the race back to back.
Yeah, the F1 Longtail is fantastic. Absolutely loving this car and with the payouts being much better in these new events I've already made back half of the F1's cost just running Sardegna for a few hours. I'm tuning up the F1 road car to give it a go next.
As to OP's question: any Gr. 2 car will do them, and Gr.1 that can be de-tuned to 800pp can do them with ease. Mazda vision Gr.1 should really stomp them given it's 4wd (not sure if it can make it to 800pp though as I don't have it) Sauber can though.
The Ford GTE and Ford GT Spec 2 Gr. 3's were fun cars to run. The Mclaren P1 GTR has the speed, but I've not really tamed it yet.
Any of the super cars should win with ease as long as you add aero parts to hold more speed through corners, though even without wing/splitter the acceleration and top speed should more than make up for poor cornering speed. I've beaten the WTC700's with the BAC Mono and the Radical analogue to the Mono by almost a minute, so given a little more power they could probably win the 800's as well. These races are more about staying away from the other cars to get the bonus, and conserving fuel more than a quick pace. (to anyone not aware you can get to fuel mapping by pressing right/left on the d-pad, up/down on d-pad to adjust it. 1 is full fuel. 6 is the least)