possibly boost leak

No way!
As cherry said there is no MAF sensor on 1.4TSI . There is MAP sensor before throttle body, and another one after. Basically there is measured (not calculated) manifold intake pressure.

As for any kind of leaks.. if you have such huge boost leak in order to have such huge multiplicative trim, sure there should be underfueling on idle (unmetered air will be ingested trough the same hole that boost will exit). So there should be positive additive trim.

In this case we have huge overfueling on idle and the only reason will be much bigger injectors without necessarily adjustments. But as I know injectors are stock ones

Anyway I'll ask him to drive a little bit some evening so I can get more data also under load.
But it seems that best thing to do is to get my odis-e and to reflash it with stock file.

Ok.
Today I take the car for a ride.

It is nice looking pig

My single charged 1.4tfsi A3 sportback is a much more fun to drive.

MAP sensor looks to be correct one, according part number from ETKA.
And it seems to be ok - I can see something like 900-910mbar on switched off engine and this is good estimation for the altitude that I live.

I noticed some strange things while I was driving.
There is plenty of popcorn sound on decel while I was driving downhill. And while there is a lot of pops and bangs, on VCDS it looks like injection timing (channel 002) is completely 0.00ms. and while it is showing 0.00ms injection timing there was lambda close to 1 instead of more than 1.2 ( I don't know where the maximum of lambda probe is).

Another strange thing was that on WOT run the car refuse to go WOT
What I mean is that I can see a lot of load request (more than 200), actual load was not even close (slightly more than 160), N75 is at 100%, but throttle was between 70-80%.
Why there is such strange behaviour?
Why there is huge under boosting condition but there isn't 100% throttle.

Maybe some bad pop&bang attempt done. There are also some automatic tools for this simple thing which make big nonsense changes. Better restore car back to original file...