That's interesting, as I have spent nearly 12 months looking, reviewing, you-tubing, e-baying & visiting Audi main dealers etc, for new or second hand alloys for my S7.
The biggest problem I had was the size of the S7 break calipers and finding alloys that I liked and that offered the caliper clearance. They are standard brakes, but their size certainly limited the wheels I could consider.
The wheel offset is crucial and I've ended up with TSW Chicane 20" x 10J alloys which have a very slight concave and sit within the arches. They have an offset of ET25 and I'm planning to fit 275/35 to the front and 285/35 to the back, but the upgrade isn't finished yet. I tried a part worn 265/35 on the front, but it looked too small.
Anyway, I wish you well with your alloy wheel upgrade.