I would suggest you turn your wheels half lock and check out the clearance from tire to inside fender - its very tight in there! I personally would not feel uncomfortable doing 275 up front.
Secondly, this M3 is rather well balanced over/under-steer-wise. If you put something that large up front I think you'll induce more oversteer than you care for.
I've yet to see an aftermarket wheel in 18" that claims to fit the M3. Rumor has it most 18" designs don't clear the brakes. I went 19" BBS RS-GTs in the stock sizes, although I may +1 the rears to 275s once I trash the 265s. All hypothetical, however, since I don't have them yet (90 day back order from the Tire Rack)!