The AC Schnitzer does sound good!
Looking forward to the Dinan later this year.
Also - I am partial to Magnaflow, have them on my other car.
They have PN 15499 under development for the 550i
Magnaflow sounds good with solid HP gains.
They really nailed it on the new 1 series IMHO.
http://www.magnaflow.com/04sound/spo...011bmw_1m.html