On the M3, the rear tire should be slightly taller than the front, so do not use 275/30R19 with a 245/35R19. Ideally you would go 255/35R19 front with 275/35R19 if you wanted to go wider.
275/35 rear with 245/35 front would *probably* be ok, but the understeer would increase, as noted above. The change in width is what changes the handling balance.
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.