Hello, sorry no one has answered you sooner, I will do my best to help you.
Given what you have wrote above and on the assumption that the work so far is of a good standard,
I would be checking the condition of the pipes for boost leaks, the condition of the air filter and checking the intercooler for any blockages.
I note no mention of the map sensor being checked, cleaned or replaced and would consider this also as they can be a common issue on VAG group engines.
MAP (Mainfold Absolute Pressure) Measures the actual pressure in the manifold which the ecu uses to calculate fueling.
MAF (Mass Air Flow) Measures the amount of air entering the air intake and the temperature for density, gives the ecu something to estimate what boost it should see from.
I hope this helps you, please let me know if you need anything further (please note I am uk based and on gmt)