A GXP has a curb weight of 3632 lbs. A hardtop WS6 has a curb weight of 3397 lbs. Thats a difference of 235 lbs. They have killer gearing, especially for highway rolls. They dyno more HP at the wheels than a GXP makes at the crank. Torque, lets not even mention that.
Weight
WS6 wins
HP
WS6 wins
TQ
WS6 wins
Gearing
WS6 wins
There is nothing that shows there is even a snowballs chance in hell of of a GXP winning that race. Either it wasn't a WS6 or the guy wasn't even trying.