[Watsey]

More info would be useful. Where did you have the dyno run ? Have you had a full diagnostic via the OBD ? Could be a failed sensor, i.e. MAF/MAP, although faults seem to be rare on these units especially at low miles. Have you checked all hoses for boost leaks, checked the condition of the air filter (should be clean at these low miles), and made sure that you're getting sequential boost from the two turbos ? Also worth doing simple things like having the Sports Gauges on the iDrive and seeing what happens as you add throttle through the rev range (including full throttle and kick-down).

290/419 is low, especially as BMW is usually conservative in its published figures. 85bhp transmission losses also sounds very high. The engine may be tight and they do loosen-up, especially the diesel units, as the mileage increases. At this stage I'm doubting the dyno results, BUT, there's nothing like the butt dyno and comparing with spirited driving with other cars as a real test off the car's performance. Have you got a dyno graph that you can post, including the dyno settings (AT, IT, gear - the 30d/35d should be run in 5th, etc) ?

My 330d (RWD) made 274bhp in stock form at ~25K miles. Now 350bhp/525lb.ft with a DMS remap, and completely reliable over the past 60K miles.

335d units are regularly producing 370-400bhp with a decent map but obviously you'd need to get any problems resolved first.