Topspeed:

..................Porsche 991.2 GT2 RS MR: 6:40.33
.......Mercedes-AMG GT Black Series: 6:43.62 (20.6Km) (fastest stock car)
..............Lamborghini Aventador SVJ: 6:44.97
.........................Porsche 991.2 GT2 RS: 6:47.30
.......Mercedes-AMG GT Black Series: 6:48.05 (20.831Km)
.Lamborghini Huracán Performante: 6:52.01
.........................Porsche 991.2 GT3 RS: 6:56.40
.............................Porsche 918 Spyder: 6:57.00
Lamborghini Aventador LP750-4 SV: 6:59.73

Edit: all other laptimes are for 20.6Km. So about 4.5s difference.

https://www.youtube.com/watch?v=9--oZuqtlFY

 

AMG has obviously paid this douche bag !  listen from 5mins Smiley he said GT2 RS MR is not stock hence the time set doesnt count