This question is like poking a hornets' nest, but I'll just throw in my objective two cents. This is speaking for a stock motor:
-Filling up with 87 every now and then won't hurt anything. Probably shouldn't make it a habit, though. From what I remember reading it might cause excess gunk and deposits, which these engines already have issues with.
-Modern ECUs can adapt, it just retards timing (maybe lowers boost?) to compensate, so it's not gonna hurt anything short-term.
-By the numbers, it won't make as much power and will reduce efficiency, however:
-When I've done it before, there was a negligible effect on MPG, and in everyday driving it didn't feel any slower. If somebody snuck in 87 without me knowing I probably wouldn't be able to tell the difference.
-That being said, I still use premium (93 where I'm at) the vast majority of the time, and sometimes mid-grade (89) if I really need to stretch my money at that moment. VWs are really finicky, so if they say you need it, you're best off just doing it.