First - as a previous comment hinted, check that your boost control valve is clean. Cleaning out the carbon in the boost control valve will restore some pep, and ought to be done about once a year if you drive the car a lot and it has high mileage.

Second - Fuel pump timing. I have no idea how to do this, but while I was overseas I had my 300CD taken to an independent mercedes shop that suggested this adjustment. The car runs like never before - it feels like a completely different car. Nobody else even suggests or mentions adjusting the fuel pump (or fuel compressor as it may be called) timing. Instead, shops usually give the same un-helpful advice you've likely received: "Well, ya know, those cars never were very fast." Find someone who knows how to do this and you will be pleasantly surprised at how much better the car runs.

Good luck

__________________
Gil

2004 ML350
1984 300CD; Ivory (sold)