By the way, could the performance issue be, because of the pump timing? By searching the net a little I found out others with m-tdi pumps had powerloss due to bad pump timing, it should be more advanced? more aggressive?
Your timing needs to be right, with the right pump. You have the wrong pump on your engine. "Aggressive" has nothing to do with it.
Read the link that jtanquay provided. It talks about the pump you need:
http://www.vwdiesel.net/phpBB/viewtopic.php?t=12912Then go to the proper page on this forum and identify your engine as an MTDi:
http://vwdiesel.net/phpBB/viewforum.php?f=5And ask for the proper timing spec for an MTDi.
There also may be some one that has a spec for running your engine with the wrong pump (the one you have) and achieving acceptable results.
Also, as Jimfoo pointed out, you need a bigger exhaust system, from the turbo, all the way to the point that it exits the car. 2.5 inch / 64mm would be a minimum. Muffler is optional.
MTDi engines have been done many times befor. The is pleanty of information available on how to do this right.
If this is beyond your capabilty, motivation, or means, sell the car and disclose that it has the wrong pump, and give the new owner the information to make it right.