As discussed in great detail prior in the forum...
The single best bang for the buck (or bang for the mileage) thing BMW could have done differently for the M3 is direct injection. BMW M seems almost capable of fortune telling. They knew they did not HAVE to include DI to have such an award winning engine. Other than the extra cost, complexity, engineering effort and departure from the base M5 engine, DI would present no downside.
Heck, it would have provided more torque and power as well. But of course, that was not required to make the car the best in class either.
I did not sway from buying the car due to mpg but I sure would have liked a lower consumption car.