As you spoke about the German TÜV, the only way to get a bulletproof TÜV certification including a legitimate emissions certificate is either putting the car into a test labour, or use a complete turbo kit that is sold by ISO 9001 certificated company.
You are lucky your car is of EU2 emissions class, otherwise OBD2 is a must have and ALL standalones are useless anyways.
Additionally an EU4 class emissions pass is nearly impossible with the turbocharger extracting too much heat to get the catalytic converters up to temperature.
Thats why superchargers are often used here in Germany, you can keep the exhaust side stock.
Just because some tuners "make it work" doesnt mean its well done. For example, every boosted M52-M54 BMW engine had the MAF linearization scaled down and never knew real world values until 2017. This can somewhat be compared with the 5120mbar hack from ME7.5.
Afaik there are hardcoded load limits in ME7.1.1 that require program code changes, so its not only tuning, like changing calibration values.
You could take a look at Bosch Motorsport ECUs, like MS4, but still, that wont eliminate the shady TÜV engineer you need for that.
Or sell your Audi and buy a BMW, our 6 cylinder ECUs can be boosted 
Yeah I am fairly sure he can forget about legitimacy with turbo conversion lol, unless you got serious connections or money I dont know. I have done similar thing with my car ( being 1.8 not vr6) because the laws dont allow for simple engine swapping here, however I am technically allowed to change everything about the engine except I have to keep the original block, this is ofcourse if you care about that stuff. IMO once you start tuning you move into grey areas with laws lolz.
And yes like Marty said there are load caps , also some other calculations are different cause of SY_turbo ofcourse.
IMO , his options are : standalone so his "tuner" can tune it or paying someone serious cash to do it if you wanna keep everything factory like, if not you got the DIY option but you will have to accept the compromises.
