The choice of narrower tire than you can actually get on there has to do with straight-line stability (a wider tire wants to follow camber changes in the road more than a narrower one), unsprung weight (wider tire + wider wheel = more rolling mass = the worst kind), replacement cost, etc.
Most buyers of M3s will never even bring the stock tires to the limits of adhesion, thus the extra width would only be a hindrance. If you go wider in the back to gain straight-line traction you have to go wider in the front, which drastically affects steering feel, weight, etc.
So they wind up (as with everything else on a performance street car) having to compromise. I think 245F, 265R is a good compromise.
The E46 M3 came with 225/45/18 front and 255/40/18 rear, but guys can fit 255/40/18 front and 285/35/18 rear with no fender mods at all. It all depends on what you want.
I am glad they will have 18" standard because it will keep the weight down more (-2-5 lbs per corner). The fenders will leave plenty of room for anyone to "Bling It Out" with 20" rims wearing 265F, 295R. Hell, you can even get some spinners if you want!
