You are right about the C7.R.The Group 3 corvette is not a GTE its a fictional GT3 specifications car based on the road going C7 stingray you are probably thinking of the C7.R the C7.R however has less power and its quite a bit lighter , the 911 is overused because its the best GR.3 overall just like the 919 Hybrid in GR.1.
You said the problem in GR.1 is not the performance difgerence
Ok lets see the advantages of LMP1 cars against GR.C and vise versa
LMP: Full time AWD = Better stability , Better traction out of cornerns , better tyre wear.
Hybrid system = better accelaration , better fuel economy
(and better starts as a side note)
Better Aerodynamics
7-Speed gearboxes
No turbo lag
Group C Cars: More power (in game)
Better top end speed
and better sound i guess?.
For the record i saw a group C car on a daily Top 10 leaderboard only once , so i thinks there is quite a difference.
I still don't agree that the RSR is the best Gr. 3 car all around. It is a good car, but when it was only a tiny bit worse on BoP (I think it was only 2% less power), it was absent from top 10. People like it and are used to it, that's all. Me, I am not able to get a personal record time on any track with it as I am used to the AMG and it suits my driving style better.
It's not that lap times are simulated very well in GT Sport, let's face it, it's not a full on sim game. The difference between GTE and GT3 is to small to be simulated correctly by GTS.
Concerning Gr. 1: Yes, exactly! That's what I mean. The LMP1s are so much better in terms of cornering and acceleration that if you would try to make Gr. C competitive, you'd have to give them even more power which would screw up BoP even more. If both had similar drivetrain and aero concepts, you'd just give more power to one of the two and your BoP is alright.
But that's no issue with GT500 vs LMP2. GT500 are a tad faster, have about 650hp, LMP2 have 600. No hybrid or AWD involved. BoP would be easy here.