There are a lot of quite good ECU flashes for the n35D engines here in Europe. However, I would never consider a 335D register turbo for the e90/e92/e93 series. Diesels have a narrow power band ( thats why they are all steptronics ), they sound not good at all and will never behave like a petrol gas engine. Probably the best application for diesels are heavy cars like the X5, trucks, etc. I owned a performance tuned X5 3.0D for almost 2 years, my best day with this car has been when the dealership bought it back and delivered my 335i ( the new owner who bought my car likes the X5 tune
) .
A Vishnu V3, Dinan or Noelle tuned 335i will beat every performance tuned 335d in any performance and / or fun related discipline. Fully warranted, in case you like to.
My first (BMW) diesel has been my last diesel. A lot of more HPFP issues compared to the 335i. Great cruising milage is not everything. The price of the cars ( 335i / 335d ) is equal here in Switzerland. The quality of the diesel in the US seems to be another problem.
Cheers,
Eugen