You really have no reason to speak on the matter. Knowledge is key. Your saying a basic stock GT with Spray will run mid 13s... That is about right.
But considering the GP at the heaviest weights in a meager 3,346lbs vs the heavier Mustang GT w/5spd @ 3,375lbs, you can't say its really heavy. A stock vette is only a couple hundred lbs lighter, it comes down to power and traction.
His GTP was cammed, intercooled, and spraying too, there are plenty of 11 sec GTPs out there, 12s aren't hard to get to with basic bolt-ons and a good driver.