I don't know if anyone else has heard about this yet, but apparently SVTPerformance got their hands on footage of the '13/'14 GT500 on the 'Ring. The time, though it bests the ZL1's 7:41.27, is iffy since they added the timer themselves; they've also posted comparisons with the ZL1 and Z/28 (the first 7:37.47 lap).

For the Camaro's part, Al Oppenheiser claims that the Z/28 ran a 7:31.9 lap in dry conditions, but it's unofficial since no video was taken of said lap.