The CF roof on the E63 M6 shaved 10 lbs off the roof (and yes, that's the best place to have a savings).
Remember that the M3 coupe (E92) has a CF roof but the M3 sedan (E90) does not as it typicaly comes with sunroof (in the U.S. at least). BMW has found that 4 door sedan owners expect a sunroof which is already a weight and center of gravity hit, so CF is sort of meaningless on a sunroof car (except for coolness factor of course).
Having said that, the new F12 6 series cars (640 & 650) are reported to come with a plastic composite roof (but not CF) and I expect the F12 M6 to have a CF roof again. F14 Gran Coupe M6 (if they build it) may or may not depending on if a sunroof is available on the M6 variant.
So, back to the F10 M5...
If they do offer a CF roof, expect it to not have sunroof and to not sell that well compared to a steel roof/sunroof equipped M5. Just my guesses...