for those of you looking for a custom Downpipe see this thread
Forum member 747flyboy has had both see post below
from "http://f10.5post.com/forums/showthread.php?t=1177153"
"M5 OEM Down Pipes
Check my post;
http://f10.5post.com/forums/showthread.php?t=1169845
Bone Stock OEM 2011 550i Dyno 379 WHP. OEM M5 Down Pipes with NO TUNE 409 WHP a gain of 30WHP combined with a growling deep exhaust note. When combined with OEM M5 Down Pipes and OEM M5 Exhaust NO TUNE 424 WHP an additional gain of 15 WHP however lost the beautiful growling deep tone to an obnoxious loud raspy exhaust ( I hadd to add 4 additional resonators to regain the previous tone). The above hardware combination along with Burger Tuning JB Stage2 piggy back and pure 100 Octane+Torco 497 WHP an additional gain of 73 WHP.
I believe this setup is easily above 500 WHP if I were to eliminate the additional resonators but not a valid trade since I love the way this V8 engine deeply and graciously growls.
"