RPI Scoops. BMC Filters, Block Off Plates or ASR Full CAI System, ASR Gen III Exhaust or RPI GT Ehxuast, RPI Pulley, H&R Spings and Spacers, If you have enough left over, the ESS/RPI Tune with SMG Tune. Done!
In terms of shear volume, I believe that the ASR Gen II and Gen III Race are equal to the RPI in sheer volume.
The difference is that the ASR exhaust is more of a deeper "roar" while the RPI has a high pitched "howl."
The best bet is to find some M5's that have the exhaust and see how you like it. I'm sure RPI can get you in touch with a few people that have their exhaust so you can hear it in person (the only real way to figure out how an exhaust sounds).
Your other options are the ASR Gen III Sport (which has a slightly different design making it quieter) or using the RPI "silencers" on their exhaust (reduces noise at the cost of a bit of power).
The BMW dealer will have to prove that your mods caused whatever part to fail to deny you warranty claim. As such, if your transmission explodes they can't say it was caused by your exhaust or something.
The only thing they can really try to nail you for is the pulley, and that's still a small chance. If you are really in fear, you can get a Dinan pulley, and be covered under their warranty.
But again, due to your proximity to RPI, I would just grab one of their full setups.
Id get the pulley, SMG & ECU tune, scoops and blockoff plates, remove the carbon filter, bmc filters and toyo t1r's to win a battle till 100mph for under 6k... and if there is enough left over get a full rpi exhaust... pulley=less rotating mass on the engine, bmc, carbon filter removal and scoops= best bang for your buck and can reap benefits at about 40mph... smg tune= faster shifts... ecu tune= about 10whp throughout the range... and better tires to launch better... the exhaust gives a good 13whp or so throughout the rpm range
2004 Mini Cooper S: Alta supercharger 15% pulley upgrade, Dinan cold-air intake, Skunk2 header back exhaust, RMW CUSTOM TUNE, Alta oil catch can, OMP Strut brace, Eibach pro-kit springs, H-Sport camber links, 19mm Rear sway bar w/ adj. end links, 18 in. Breyton mini magics, 6lbs lighter crank pulley, polyeurathane tranny and engine bushings, wheel Studs, Wilwood BBK... More to come soon!
RIP=White 944 turbo: KKK K-27/6 turbo, upgraded intercooler, Lindsey Racing hardpipes, Autothority MAF, B&B exhaust with 4.5" oval tip, Siemens #55 fuel injectors, Lindsey Racing MaxHP custom Chip set, 3 bar fuel pressure regulator, Re-worked head and slightly overbored pistons, Spec Lightened 6 puck stage 3 clutch, Quaif torque biase differential, Apexi AVC Type-R Electronic boost controller, HKS type-1 turbo timer, Shimmed wastegate, Champion Motorsports adj. bypass valve, Weltmeister front and rear sway bars, Weltmeister strut tower brace, Koni adj. "yellows", unknown springs, Turbo Twist rims, Kumho Ecsta Supra tires... And a couple of more unknown mods...