Despite what some people want to believe, these cars are very similar in performance when matched up in a roll race. As ON D BIT mentioned, it's a drivers race, but really comes down to how fast the GT500 driver can shift. Given the long gearing in the '500, he should only have to shift at most twice.
Its more of a GT500 race. Its an auto, once the HC goes WOT he has nothing to worry other than the road ahead. The GT500 needs to make fast shifts in the right RPM otherwise I can see the HC taking it.