Anyhow regarding 20" rims...
I am living in Norway, and usually we use winter tyres from october to mars/april. Today I did put on my 20" summer tyres. Many posts in this forum has been regarding how firm the Q5 S-line is, so I have been a bit worried about this, but after a short test drive, I must say I do find the 20" 255/45 to be perfect for this car, both looking and driving. My winter tyres is 17 235/65, so the difference was huge. Can't help it, I just love this car.
My tyres was preset with a very high pressure from factory, but I have now decreased it to the value described on the sticker on the door.
Could I ask what pressure you have used and is the wearing equal on the sides compared to the middle area?
(sorry if my english here is not perfect)
3,5 mm seems to me the limit for a safe behavior on very wet roads
in Belgium, its rains to frequently for toying with safety, and the roads are far from perfect, even the highways (track shaping)
I expect to have my tires replaced after 20k miles, after a switch front/rear at 10k miles (front tires wear a little bit more on the edges due to the over?-assisted steering...