After 300 miles in the 6 Series GC 640D M Sport ( same size as the M6 GC) I can say, the boot is 1" shorter with the same width,my M5 boot liner goes in with a slight turn-up at the front.The rear nee room is much better than my M5, and rear seats are good+ they fold down.
M-Comfort seats are required (for me anyway) as the STD Sport seats are crap.
NO problems getting it in the Garage.
Over all Me likes.