PP isn't a real-life stat, therefore you can't measure it on cars in real life and then transfer that finding to a video game.
If you're willing to judge by the cars in the game that are GT1/GT3, then take them all and estimate the average. Only cars that I can think of at the moment are the McLaren F1 Race Car, the Corvette C5-R, the GTS-R Oreca Vipers, and the like.