There's an almost $25k difference in price in the US between the 550 and the 650GC. I'd expect that difference to carry over to the M models. That's hardly a rounding error for most people.
Current - '15 Porsche 911 GTS Cab 7MT | '16 Audi S6
Retired BMWs - '13 F10 M5 6MT | '08 E60 550i 6MT | '04 E60 545i 6MT | '98 E39 540i 6MT | '94 E34 530i 5MT | '84 E28 528e 5MT