I have driven GTIs with 17" and 18" wheels, with and without ACC.
My answers below relate only to the ride and not handling. It was not possible to really try out the handling prowess of the car in London. Tyres were Bridgestones on those cars. It was quite difficult to remember how each suspension type and suspension setting felt and compared as I tried the cars on diffirent days.
17"
Normal ACC mode was very similiar to standard suspension, though the former felt very slightly less jiggly. There was not much in it, IMO.
Comfort mode ride was quite amazing and could be classified as close to a magic carpet.
Comfort mode was not as well damped as normal mode or the standard suspension and could crash occasionally on sharp ridges. However, comfort mode provided a superior overall ride to normal mode and to the standard suspension, IMO.
Sport mode for me was too uncomfortable on London roads.
18"
Normal mode felt better than the standard suspension and I felt it was not acceptable with either suspension on London roads. Comfort mode was really the only solution for daily driving and produced a very acceptable ride though nowhere near as comfortable as with 17" wheels.
Again comfort mode was not as well damped as normal mode and could occasionally crash on sharp ridges.
I use comfort mode all the time .
Sport mode was uncomfortable though not significantly worse to that with 17" wheels..
Overall, 17" wheels were better for ride and, from what I can remember, produced slightly less noise. But they looked/look awful in Monza form. However, Herr Doktor's 17" wheels look fabulous.