The current GTR cars are more akin to FIA GT2 cars in performance. My suggestion is to use modified versions of the current GTR cars to create a new GT1 class. This is based on an idea on Wheel4Hummer's idea in the LFS Tweak thread.
Guideline specifications for all cars
Power : ~625 horsepower
Weight : Unsure, presumably about the same weight
Traction control : Possible on one or two cars.
FX1 : 3 litre I6 turbo.
XR1 : 3 litre V6 turbo
FZR : 3.2 litre boxer 6 turbo.
Future expansion room would be cars based on any supercars.
Guideline specifications for all cars
Power : ~625 horsepower
Weight : Unsure, presumably about the same weight
Traction control : Possible on one or two cars.
FX1 : 3 litre I6 turbo.
XR1 : 3 litre V6 turbo
FZR : 3.2 litre boxer 6 turbo.
Future expansion room would be cars based on any supercars.