The dealer should be absolutely able to fix the sticky vanes,,, but they have to take the turbo off and the housing apart,,, so quite costy.
*IF* the spray thing works then it should remove all build up in the intake [turbo, IC and EGR included] and thus leaving you with no need to remove anything.
IIRC in england it is called 10k diesel booster,,,
What I fear is the debris falling into the engine or getting stuck with the valves,,,
There's a thread about that in the tdi tuning section but there ain't much info in it yet,,,
Many thanx for the N75/N18 vs blackbox info,,, I indeed had doubt about that since I saw a golf which clearly had a pd engine but had the N75 and N18 separated,,, I thought that was a 115hp but then the alloys were different,,, now it's all clear,,,
The overboost prob can be a bunch of things,,, first to check are N75 and sticky vanes,,,