I personally dislike how Nürburgring times are so trendy lately. Hell they just put the Cadillac ATS on the Nürburgring. You can track tune a car on lots of other tracks. I think there is a large publicity factor in going to the ring too IMHO, because there are plenty of relevant tracks you can develop a car on. But since everyone else is running the 'ring you can use it to compare your car to another car. At least the Boss 302 used good ol' Laguna Seca for their development.
The vast majority of people who will own a 2013+ GT500 or ZL1 will never set foot in Germany, let alone be in a position to drive their car on the ring. And for those that do, they won't get anywhere near driving a sub 8 minute lap there on a public day there.
The guys that are driving the ring have usually circled the track dozens, if not hundreds of times. They are familiar with all the turns and lines required to navigate it professionally. It not like replicating a 1/4 mile time here.
But that said, if the 2013 GT500 in fact pulled a 7:3X on the ring, I have a very difficult time believing they would not publish such an outstanding time eventually.