I'll repeat, DO NOT put a K&N on the D/A!
As for your original q, I just put a MBRP 4" DFP back with a muffler delete. I needed to clear a rock guard: http://www.wakeworld.com/MB/Discus/messages/65921/547051.html
It looks a lot better, but there are no performance gains that I can tell, have heard that it'll drop the egts down 50 deg or so, but I don't have a guage yet. Can hear the whine of the turbo better, but it is not louder. Was easy to install, backed onto a set of ramps and got the original piece out w/o cutting it for future warentee work in needed.
(Message edited by siuski on May 13, 2008)