Fully modded Zonda R is pretty nasty. The Corvette ZR1 Race Modified is also a pretty quick ride when it's fully tuned, however, I don't know if its quick enough to compete with the Group C/LMP cars. Your best bets though are the Peugeot, Audi or 787 as mentioned above. I'm hoping that more of these cars like the Speed 8, GT-One, LMR, R8, 905, 88C-V, C9, and XJR-9 are made premium in the forthcoming updates.