The tuner recommend it by default, because manufacturer does.But it isn't EXACTLY the same motor now is it? Different rings, different oil pump, there is just enough different to warrant different required oil. Yes one can argue the 0w20 oil is for fuel economy, I would agree. Yet this doesn't explain why APR and Unitronic both recommend the use of 508 in their stage I and II and some Stage 3 Builds??
Oil now a days is much better than it was. Heck Formula 1 race teams run a 10W in some of their cars, granted they get rebuilt after every race, but the point being, there is an assumption that the Ow20 can't protect the engine? I disagree with that. If you track your car? Get an Oil cooler, if your engine is totally modified with pistons, rods and what not?? Then go by what the builder says. Stock everything? Use What VW says. If your Tuned, use what the Tuners recommend. At this point. it all points to 508 for 99.9% of the folks out there.
0w-20 would be water by the end of a track day.
For street, no reason to run anything but what is recommended. That doesn't mean 5w-40 is going to harm your car.