Well first thing you'd want to do is to actually set those LMP, an Group C cars to their listed power levels from the showroom, I did that to all of my race cars as that monstrous power boost they get is utterly ridiculous. Once you do that then you can gauge performance and tailor them to be quite close to one another. They will all never be equal simply as that doesn't happen in real life either, but you can get them relatively close as long as you can handle each one's quirks and nuances.
I have built up a garage of about 13 GT3 level street cars all running the weight of 1350kgs and no one over 592Hp and all must reach 580pp with that weight limit and also all cars must have a rear spoiler and flat bottom attached(without cars don't reach the PP limit and will top out the hp figures as have strange top ends). Thing is I run everyone of them on sport hard tires as for me these are the best racing tires anything else is a bit overly grippy. I ran all the cars on Red Bull Ring and they all performed around the same going around the course getting within two seconds from fastest to slowest. There were some eye openers like the Camaro '10 which is one very solid machine and the M3 '07 which is faster than it actually feels. I had to eliminate a couple of cars because they were way too fast for their power, Ford GT '05 and MP4-12C MR cars are just incredible but these two are at least two seconds faster than the fastest FR cars. Not sure why but it's actually fun to tune as I was readying myself for GT6 eventual community update so I can run a street car GT3 spec series, but it will only work if the rules are very detailed down to the tuning allowed and parts that must be installed to enter to ensure that things fit within the limits.
