If so, any insight as to how much worse (if any) the car runs with 18" winters over 17" winters?

Any feedback would be appreciated.

haven't really ran them in heavy snow yet but wet & dry they ride & perform just fine.

I've been in some significant snow without any issues. I used to run 16's on my A4 in the winter and only notice a slight difference (but that could be the car as well). IMHO, you'll run into clearance issues before the 17 or 18 size becomes an issue. I've only been stuck once and that was on a unplowed side street after a 23" storm.

<img src="http://pictureposter.audiworld.com/5752/dscn0377.jpg">

Work fantastic in the Boston winters and the Avus wheels are very rugged for potholes and such.

My 19" RS6 wheels got wrecked in the roads up here!

negligible, at best.

I'm on my second winter with the WinterSport M3's, and they rock.