Supercharged? Yikes! That in itself isn't my concern but suggests you or whoever put the kit in undoubtedly altered the stock DME fuel mapping (chipped or flashed the parameters), changed out the injectors, etc. This movie just changed as more variables from forced induction are present. Yes, seek an independent shop with experience with German cars. The base-line data for normal cars is useless in this case they should understand that. A tuner type shop may have a better clue what the affects and readings are from your modifications but your symptoms may be simply related to what stock engines would suffer from.
My concern is, if you have no history who/where the supercharger was installed you may be fighting someone else's problems. Let's assume all was performed correctly and program/injector alterations are healthy. The issue likely is external to this but you never know? The other thing is, whoever installed this probably drove the car hard racing around and now you're getting the pleasure of replacing what they hammered. I hope you can find assistance with this but certainly be aware some shops may not receive your car with a positive approach. I'd make a few calls qualifying the situation and don't accept the "well, we would have to look at it" replies. Service writers, you typically get on a call, just want to take your money to tell you they don't have a clue after the fact. Try to get to the actual mechanics to discuss the problem. Sometimes going to the shop is the best deal. They often will respond in person better than on a phone and you'll get a feeling about the shop looking around at the cars they service and talking directly with the mechanics.
Good luck brother.
Anything made can be made Better