The "voided warranty" is a myth perpetuated as a scare tactic. No, BMW won't work on your exhaust if it's aftermarket. But they cannot legally refuse to service your car if you modify it.
Please read this article very carefully:
Mods & Warranties - How YOU are Protected
Also, Supersprint makes a great product, but it's pretty quiet. If that's what you're looking for, and you don't mind the relatively high price, they're awesome.
Personally, I like REMUS systems because they're easy to install, really high quality, and affordable.
We carry all of it, so feel free to ask if you have questions. Toll free.