I was thinking about using some 19" RS4s (ET42) with 245/35 R 19 front and rear (quattro) on my S-line avant but fear the ride will be a bit harsh on some of the roads I use. TT S-line on 35 profile 19s seems to be more compliant, but they're 255s.
AT ET35 not sure on how much clearance you'll have assuming you haven't lowered your car. In diameter terms, you could even go to 255/30 R 19 on rear as overall diameter is not as great, although you'll have a difference front-back with implications on quattro. Haven't got a clue on clearance at that level but at least it will spark some debate...