If your really concerned about MPG, try out a 335iS. My roommate just picked up an 11' E90 6 speed and the dealership had a beautiful 335is 6 speed in the showroom - Alpine on black. One of these would be nice with an LSD, downpipes - tune (380whp).
To give you an idea going 70-85mph my friends M3 returned 20mpg. I would hands down only choose the thoroughbred but the 335iS is nice if you want the daily driver aspect/torque factor/comfort