My personal take on this is, this isn't an actual race. It appears to be a track event with people and their personal cars. They aren't going to tailgate each other, and there's no way the GT was going to have to wave the ZL1 by. So I believe the pulling away, while it could be certainly capable of doing so, was attributed largely to the courtesy and save driving of the event. Again, the GT may be able to legit pull away like that, I think it was more due to safety and courtesy for the track event.
As for exhaust sound, it's stock. The owner is here and explained the microphone for his cam is in the rear:
http://www.camaro6.com/forums/showthread.php?t=501648