I have the 18 monza shadows and although when I was first thinking of buying the GTI I thought these looked horrible they have really grown on me. I think they make the GTI stand out from cars with normal alloys, at least the monzas are somewhat different.
Why would you want 19s on when the ride on the 18s is supposed to be a lot harder than the 17s, although it is not something I've found having been in both I much prefer the 18s.