Thanks for the input guys.
Some of you are not getting what I mean when I say custom. What I mean by custom is taking it to a shop and having them build one for me instead of buying a system straight from the factory already specifically made for the M3. Remus, eisenmann, etc. ...just to clarify. But then again, I guess a Remus or Eisenmann exhaust could be considered custom as well.
My main concern by taking it to a local shop and having a custom exhaust built is whether or not I would lose performance in any way...