Since my BMW muffler has rusted out and fails to pass inspection it's time to find a suitable _performance_ replacement. I like the throaty sound of my rusted out muffler (aka a "hood-rich" performance muffler).

TC Kline sells Supersprint and Stromung but really recommends Stromung as slightly improved throughput on their dyno tests. Only downside is that they are custom built at time of order and take 4-6+ weeks to arrive.

http://www.tcklineracing.com/OPC/exhaust.html

As a side note, also ran across a site that debunks many of the BMW snake oil upgrades...
http://www.bmwe34.net/e34main/upgrade/Recipe.htm