I'm having similar symptoms after turbo rebuild (bought a rebuilt core) moths ago (since then I have spend a lot of time reading and looking around the car searching the matter).
I'm blaming the VNT vanes because they were worn-out when I inspected them, anyway I had to re-use them due the my wallet saying "no more cash for the car". I'll replace them this month.
EGR nulled, and the car is nearly dead before 2000, then it suddenly kicks very hard around 2000+-100rpm.
I tried playing with the stop screw, sometimes it makes a bit better but then the turbo reaches an insane 1.5 - 1.6 boost and the overboost comes in.
My car has not a vacuum operated actuator, instead it has a pretty rare pression actuator that makes the vanes to move when the preset 1.1bar boost is overtaken.
Also I'm looking to the Pierburg MAF as a possible cause, because it's not the genuine part.
Tomorrow I'll test a 100% genuine and working MAF taken for working car which's owner decided to break and sell as pieces instead of selling it as-is.
My car also had a lot of power in the 2000rpm range before, exiting the roundabouts in 2nd gear was instant fun served with screaming wheels