Yes, the Mustang GT500 laptime is real 

The car seems to be and sounds 100% stock. There's even a video to confirm this.  The Ford Mustang Shelby GT500's Nürburgring Lap Time: 7:39.28? http://oppositelock.kinja.com/the-ford-mustang-shelby-gt500s-nurburgring-lap-time-7-1455203202