I'm still not convinced by the above in terms of the car over-fuelling/running rich... this is because the car in stock mode should not run rich full stop.
I agree..... in this country. But if its an ECU set up for a different country, where environmental conditions not seen here are present that the ECU has been programmed to compensate for - its a possibility. In that country, running on that countrys fuel no overfueling may be apparant. One cars meat is another cars poison after all.....
Also remember that its not sensors or instumentation showing the car to be overfuelling, its the tuner who is presumably comparing it with his experience....of UK / European cars.
Another thing to bear in mind is that when that ECU is plugged into the VW diagnostic machine, it will match up the map that should be on it with the region its supposed to be in. Not the region its actually in unless you ask specifically for that to happen.