There are multiple guys at tire rack. I am usually the one that people talk to because I work extensively with the M3's due to my involvement with the forum.
I defintely have recommended the 255/40R18 for the winter. Typically that would be the rear size in a staggered setup.
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.