I think you should RM it then fully upgrade it. It shells out over 900 HP, and can compete with Group C LeMans cars. There was an Indianapolis tune lying around somewhere on the tuning forums. It works on nearly every track. With that tune the car has fantastic handling and redlines at 240 MPH.
First race mod it, then fully tune it. That will get you a 892 hp (with oil change) RM Corvette that can achieve a top speed of over 360 km/h. You can win Like the Wind with it easily when you add full downforce.
my 900+ HP ZR1 can beat pretty much any other car in my garage! It's for sure the best value of any car I know of. Make sure you tune it well though. I have heard that if you leave it stock, it understeers like crazy and the top speed is kinda low. If you dont know how to tune, I'm sure there is something in the tuning forum.
The z-06 RM is lighter than the ZR1RM. The engine is lighter and that makes it handle better than the zR1rm but the ZR1rm is still faster in a straight line and has more grip.
RM ZR1 is better by far. RM ZR1 vs RM Z06 ain't much difference at all. RM ZR1 makes like 20 more max hp while the RM Z06 is a decent bit lighter. I prefer the Z06 RM color scheme so I use it more. Pretty much same performance in my experience between the two.
Also only reason these RM Z'a can hang with the LMP cars is because the PD's AI is driving them. Take a 787b for example and drive it even 8-9 tenths and it will destroy the ZR1 RM Indy time for example. Don't let this games tuning fool you. In RL or if driven fully by the AI none if these RM's stand a chance against any of the LMP or Group C cars.
If you're just talking the ZR1, RM it and then add parts as it's a beast of a car for the money.
As for the Z06 / ZR1 debate, I have both and prefer the ZR1 as it's a bit more raw and suits my style but you could pick either one and you wouldn't be disappointed.