With the right wheel, 295/35R18 can fit rear (some have reported going wider, but that is the widest that Tire Rack officially lists). That tire is too wide for stock wheels though.
So 275/40R18 is no problem, and does fit on stock wheels.
As Mr. Harris mentioned, 255/40R18 is fine on the front.
I have accepted a new position at Tire Rack and am no longer in the sales department. firstname.lastname@example.org
is available to assist with M3 questions.