I did a search thinking that there would be a comparison of the available exhaust systems but I could not find one. Too many choices so I am asking for help (or opinions). Milltek, gmg, supersprint, awe, and I assume there are more out there. My selection criteria would be 1. performance gain; 2. sound; 3; appearance; 4. price. I would also like to know if anyone has a difinative answer from Audi about the warranty. My understanding is if the sensors or cats are changed, the warranty may be void.