I would say it
is possible to get over 30mpg from an M3 - but its not the norm. You have to be driving pretty economically with a very light foot to achieve this (which is quite hard in an M3).
You'll certainly get 25-30mpg typically on a run and expect less around town.
But I do think for a 450bhp car it is pretty economical, given the performance. Just don't expect 2.0L economy (or even 340i economy).
As for a C63s - I don't think I ever got over 30mpg in that. Maybe once.
