There's a heavily moded (I think '06-ish STi motor swapped) carbon fibered, slammed, murdered out older style 2.5RS around here that has 3.5" oval-tubing turbo back. It is beyond words... I have never heard nicer exhaust on anything in my life. If I see him around I will make sure to take a vid.

You could always go with weight reduction via CF; weight reduction is the best mod there is.