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 SSII M3 Coupe - KW Clubsport 3-way, Akrapovic Evolution, ESS Tune, StopTech Trophy, IND Euro Front Bumper and Gloss Black Mirror Covers, TMS Studs/Lugs, BMW Performance Carbon Fiber Spoiler/Interior Trim/Center Console, BMW Edition Grills/Gills, BMWPedals Extended DCT Paddles, Mode Carbon GT4 Splitter, Advan TC3, BBS E05