Maybe try and change the oil and filter as a first step? Sometimes you can get away with it. Make sure the filter is changed - most dealers will tell you there isn't a filter.
Seems you can adjust the idling speed using vcds
https://www.audi-sport.net/xf/threads/186452/
I know this make not explain why the revs are higher but maybe your mechanic adjusted it?
Have you checked the DV as suggested above? These are known to split.
Also, check the intercooler connections (you will see oil near the connections if they are leaking).
Your symptoms sounds like a major boost leak.
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