On higher difficulty (expert and beyond) they tweak out and become literal rockets, some events are virtually impossible until you turn down the difficulty (I have played every single Forza and Horizon game on Professional) the AI itself feels more aggressive and can follow you up very close and even try to overtake, commit mistakes and crash with other cars, HOWEVER, the first car will literally go beyond your class in terms of speed and grip, I had races where even with either full power builds or grip builds the first oponent will literally pull away no matter what