One possibility to consider is that the injectors need to be removed and serviced - a poor spray pattern or faulty injector can easily cause loss of power.
The comment I made still stands - the competence of the mechanic is crucial for a good rebuild...
Edit:Another possibility, that will not show up necessarily on the codes, is that one of the hoses between the turbo and the engine has a split in it so the engine is not getting full boost - symptoms much as you describe. The best way to check - remove all those hoses and check / test them thoroughly...