Many of the wheels from 8P A3s and TTs will fit on (same PCD). There wouldn't be any initial issues apparent unless they weren't clearing the brakes or hitting arches etc.
However, most of the issues surrounding incorrect offset will become apparent much further down the line. If the offset and wheel width are different to that recommended, they're going to put different loads on the steering and suspension components. It could also cause uneven tyre wear. Audi have obviously tested this extensively and come up with recommended offsets for each wheel/tyre size.
The standard wheels run 7.5 x 18 ET51. I'm guessing that if I changed to a 7.0 x 17 alloy, the offset could be different. It'd be good if someone with 16" and 17" wheels could post up their offsets for comparison (they're stamped on the inside of the wheel spokes - you'll need your missus's make up mirror to check!)