An aftermarket exhaust system will not void your warranty. I spoke with BMW about this. Obviously, the actual exhaust will not be covered by bmw if mods were done, but the new exhaust usually carries its own warranty anyway if its a reputable brand.
If you want to get a great sound, remove the canisters/resonators. It's super easy an cheap for an exhaust shop to do. Not sure if the m6 has a stock X pipe, but on my 650 I eliminated both resonators and put an X pipe toward the front of the car. Cost $200 bucks.
2012 BMW M6 CONVERTIBLE