One thing the 'Ring does test that could help level the playing field a bit for the M3 is balance and stability in the face of some horrific road conditions (for a track) that are taken at insane speeds. 505 HP is only good if you can put the power down. The long wheelbase (for a sports car/GT) of an M3 makes it very stable so you can eek out that last bit of grip and get out of corners a touch faster. Once you get to a certain HP/weight level, HP starts making a bit less of a difference because traction issues prevent the driver from using all the power all the time.
With that said, the M3 will probably be porky (3500 lbs +) vs. the relatively svelt 3150 lb Vette with a lower center of gravity, bigger tires, more HP, more torque, and less frontal area (aero drag) so the nod still goes to the C6 Z06.