i test drove 4 GTI's - 2 with very basic spec, manual and 17"s
2 were tarted up, DSG, leather and 18"s etc
the 17"s definitely handle better and to me the car felt lighter on its feet. i still went for the 18"s though as they look a lot better
if i can be bothered i might change to some light wheels log OZ ultraleggera or something. there 5kg a corner lighter than monza shadows.