tbh the dealer should do more than just check fault codes , but also log boost pressure etc if theres a suspected issue.
However Im not surprised they havent.
From here on in I could see your choices as
1. Keep WALD and try and find a local VAGCOM owner who knows what they are doing to log some useful info , boost etc
2. Reflash ECU to std , although im not sure this is gonna help if the VW dealer isnt willing to delve deeper as youre problem may persist after that and then what they gonna do?
3. Just bite the bullet, phone JKM (prebook) take a day off and say they have a day to fix. (how far is it for you?)
Can you get your AF logs + dyno graph up so we can see its profile?
1 or 3 at this stage