Personally the first thing I would do is require the power limiter on all cars which have more horsepower than their real life counterparts.
In GT, many LMP cars have the real life HP displayed correctly in the dealership, but when you buy the car, it has far too much HP.
For instance, the Jaguar XJR-12 is supposed to have a little under 800 HP, as it did in real life. But when purchased, it has well over 900 HP without any upgrades or an oil change.
The Bentley Speed 8, Pescarolos, Toyota GT-One, and many others have this issue.
Before ANYTHING else, I personally would require the power limiter be set in such a way that these cars have real life horsepower. It will immensely balance the gap between the older and newer LMP's; currently, the Group C and older LMP's are far faster than the TS030, R18, and 908 due to this issue.
Fixing the power in such a way would at least help balance the difference between these cars. After that, I would start with ballast being added to cars which are faster than others. Should do the trick best.
