I got 25mpg avg in my M3, same engine, heavier car and I drove like it was supposed to be driven. 9.5k miles on the rears and then get new ones. My new Zed s35i gets around 25 in town and 35 on the highway with the cruise on. (300HP)
I'm with Easty on this, don't buy a performance car and then worry about fuel economy. They don't go together very well.
9,700 miles now and I will need new rear tyres soon! 