The LS2 in the GTO is overrated.
At local dyno day my friend's L99 with intake and programmer did 357 whp.
A stock LS2 manual did 323 whp, and another ls2 with headers/intake/tune did 366 whp.
LS2's really make more around 370 at the crank stock, an auto ls2 would have dyno'd significantly lower then those 6-speeds.
GTO's weigh about 3600 lbs. G8 GT's weigh around 3900 lbs.... The GTO has a 200 lb passenger(just a guess) and essentially eliminates the weight difference. From my experience close races like that are determined by whichever car has a passenger(when there's just one for the 2 cars).
The LS1 is a better more reliable overall package then the ls2 when it comes to heads/cams... It's stronger and the best years for the LS1. My friend's has 460 whp and traps 119 mph and pulls cars that trap 120+ in real life, his is the ls1.