Reguardless of warranty issues, most bolt on exhaust upgrades are for only one thing, sound. The added power is little. I will say that some do have a vastly larger increase than others, ie Active Autowerke
I haven't gotten around to installing any exhuasts on my 328i, but I did do a little searching of my own. AA was used in a comparision test (outside of AA) where AA scored the highest with Supersprint comming in REAL close. Here is the AA page of those results (the orignial page had the actual sounds from each tested exhaust unit on the same vehicle, an E36 M3), Comparision
Perhaps if you do a little googling, you may find that original page, it had the same photos in it with sound clips of each installed exhuast.
There's also UUC
$$$$$ and Remus
. Both sites have sound clips.
Hope this helps a little,