From this forum I have learned the following: as a world-wide manufacturer, BMW has spent millions on finding that regulatory/performance/design sweet spot that the majority in the world can live with/enjoy. That said, any internal combustion engine has three basic elements; suck, burn, blow. To experience any real gains in HP (and its harder than one would think with a modern BMW) you have to address all three together. Putting new headers on eliminates the catalytic converter (blow) and is a huge improvement as you skirt a major regulatory problem BMW cannot; however, you will then need to also address the other two elements, suck (intake) and burn (fuel management). A mod of this sort calls for engine mapping adjustments (power commander and others) in order to optimize them - which may affect the warranty or not, depending on where you are. If it is sound alone you are looking for, a slip-on can can change the sound but the stock apparently sounds pretty good to most. That said, those on this forum have yet to hear what difference unscrewing those baffles on the back side of the stock cans will have (which should not affect warranty I suppose). Still waiting on that brave soul to get that special "tamper proof" torx wrench to open her up to find out what is in there. Bottom line: stock works damm well for most, if you are one of that needs more, you need to go all in, otherwise you wasting time/money unless you want more tenor and less baritone or vice versa.