As a rough guide, the offset should be within 5mm of the original. Beyond that and one is changing the steering geometry, but I have no idea whether you would feel that through the power-assisted steering.
I have moved from a set of 20" to set a set of 17" Audi wheels - in other words an extra 1.38 inches (up from 3.5" to 4.88") of tyre sidewall. I feel that the smaller tyres give a lesser thump when traversing potholes, beside being less vulnerable to kerbing. The improvement may be greater on your Allroad with air suspension. The wheel/tyre combo was purchased from eBay seller "germanwheelsdirect" in Northern Ireland.
The 20" wheels look fantastic (and are great for the showroom guys), but I am sure the Audi engineers intended the A6 to ride on 17" or 18" wheels.