Ok, so after talking with some Engineers and some of you on the forum, I've found out that just changing mufflers won't neccessarily yield any performance gains. I was told that the only way to get gains strictly with a muffler swap is if the muffler is the most restrictive point in the exhaust line. I have a 2000 323i and I'm not sure if anyone knows what the most restrictive point would be on my car. I'm asking because if I'd like to know what to replace in order to get the best performance gain, be it from manifold/headers, cats, or muffler, etc. Thanks!