I have thought about referbing a set of 17s to look like the 18s and so have the best of both worlds as im sure not many will be able to tell the difference from a distance.
I found i got far less bump steer and kickback with 17s over the 18s on the mk5 and mk6.
IIRC the topgear laps the 17s were faster than the 18s on the mk5. Also when VW launched the mk5 GTI if they knew it was going on track the car was normally supplied on 17s.
VW also know that from experiance polished wheels cause lots of warrenty returns so if they don't supply every car with them it will cost them less under warrenty