The correct procedure to verify fuel delivery is to observe the amount of fuel delivered by the lift pump in a specified time.

Sounds like you have multiple symptoms here......

Having the injectors tested for opening pressure and pattern is never a bad idea but I'd start by trying to document just how bad the lack of power is - have you timed any 0-60 timing runs? A good running 2.5 turbo will run about 11-12 seconds. A 2.5 turbo without boost will take 18-20 seconds to get to 60 MPH. If your acceleration times indicate lack of boost I'd suspect a plugged ALDA line, a lack of vaccum to the boost control system or a bad vacuum control servo (in that order of likelyhood).

I've got to admit I'm not actually sure what "nailing" sounds like - the term is thrown around here quite a bit but I've never had any of my MB Diesels emit noises that sound like the descriptions I've heard here described as "nailing"