According to this article the new F10 M5 is getting on average 24 MPG:
http://www.worldcarfans.com/11106153...ially-unveiled
I figure if a Twin Turbo V8 is getting on average 24 MPG, then a V6/I6 Tri/Twin Turbo (or whatever the engine variant) should be getting better than that [I would hope] or the same.
So, if I were to put down a guess, I would say the new M3/M4 should be getting an average of 25 MPG, conservatively.