You cannot dismiss the alda line etc. so easily. Jettas even have a fuel enrichment vaccum or pressure line from turbo to pump. Happens to be almost trouble free system on vw but not so on mercedes. The primary reason for the slow build in revs and low overall top revolutions is fuel starvation. Even though you hear the turbo spinning it is just increasing the air into the cylinders with no corresponding or minimal increase in fuel if the enrichment system on the pump is not recieving the proper signal.. Do not rule out things like rupturered diaphram or obstructed lines as a few have mentioned they are the really common causes of your concern if the fuel filter does not do it.