Nothing scientific here but in Longboarders Airstrip compilation video, you can see both a stock F10 M5 and a stock 13 GT-R going agaisnt Longboarders VT2 and the M5 seems to give him a better show! Still goes to show that if in fact an M6 is faster then an M5, it should also be faster then a stock GT-R!
vs GT-R @ 2:30min
vs M5 @ 6:20min