Here's some info on the resonated version that can also be offered ->
http://www.custom-performance.net/pr...t.asp?idpage=5
M3 guys love it because it kills the S54 rasp (heh, they call it "The Rasp Terminator"). Granted we have dual cats in our S1 but I still see this option as being sought after considering the price the M3 units sell for ($459 shipped).