That's not a 700+ whp GT500. Maybe it dyno'd that on race gas, but there is no way it was making that power when racing the GTR. The SN95 Cobra with a lot less power looked to hang better.
i guess that GTR made 585 on a mustang dyno and one that reads particularly low at that. it also outran a bolt-on/150 shot C6Z. Not really surprised by the outcome, especially considering the cobra was likely spinning at the lower speeds and just cant shift like the GTR can.