Get an aftermarket exhaust. You seem to be overly paranoid about your warranty. I would not worry 1 second about my warranty if I changed the axle-back exhaust. That's not going to remove the warranty on your entire car. Only impacts the exhaust warranty...
'13 M3 - Silverstone II - Sold
'04 M3 - Jet Black - BBS E88, KW Clubsports