If you want to add on some "non-standard bits" such as antiroll bars/rear sub frame bushes/ downpipes I wouldn't go anywhere else but All Fours Garage
Andy is the main mechanic there, though he isn't a official BMW tech (the garage is actually a CitroŽn and Peugeot specialist
) he can fit/fiddle with just about anything and has access to all the needed coding gear, his previously fitted a new turbo to a 330d and I've seen 911s been worked next to all kind of run-of-mill cars.
I've been using him for 5 years+ firstly for my old Nissan 350Z, and now my 335i. He'll even lend you his daily drive whilst he works on your car...its an all powerful Citroen Xantia, which has ride like a sofa compared to my M sport RFT shod 335. He also has a single turbo RX7 which he keeps hidden away
I did approach a few BMW independents about getting my anti-roll bars/M3 subframe bushes/downpips fitted but none ever got back to me, it seems most didn't want the hassell of doing job they hadn't done before...I've already warned him that I'll almost certainly be be back to him for some new turbos at some point