2007 335d standard. Thought i was going mad as the power depleted over weeks possibly months. I have owned her since Jan 2012 so i remember she was quick when i got her, but you obviously get used to the power drop off.
Anyway, power went pretty much completely from Thursday last week - the usual nothing until 3000rpm then warp speed.
Read the various posts on here and concluded probably pressure converter, possibly red boost hose (as it had oil on the underside) and remote possibility a fractured vacuum hose.
Used the guides on here to expose the pressure converted and noticed one of the small braided hoses the goes from a T piece into the pressure converter looked very ragged at the bend - I prodded and sure enough it was worse for wear.
Now I am "Mr Lucky" when it comes to pretty much everything. Last week I got a letter from Readers Digest saying "unfortunately you havent been selected for this months prize draw". lol. So you can imagine my doubt that this was the cause, I wasnt even sure if it had fractured. Anyway after debating I thought while I have some spare silicon boost hose the correct diameter - what the heck. The piece measured 10 cm or so.
Replaced it and bingo - I have a new car again!
Big thanks to Mistryn et al for their threads etc that allowed me to do the investigation work that fixed the problem. Suppose i should replace all the other hoses now
Hope this helps someone else with a similar problem.