That pretty much sums it up right there, especially when you have to consider staggered front/rear sizes. Maintaining front to rear proportionality adds yet another headache.
255/30R20 and 275/30R20 are a little wider, not too much taller, and will not rub (assuming wheel offsets are reasonable). Max wheel widths would be 9.5 Front and 10 rear.
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.