Thanks to the high offset, you would need to add spacers to keep the inner edge of the tire away from your strut.
Based on the wheel dimensions, the inner edge will be ~17.4mm closer to the suspension strut than your original wheels/tires, and the outer edge will be ~4.7mm further inward.
You will need to adapt the hub bore from the 57.1mm hub on the RS6 to the 66.56mm bore on the wheel.
You'll be in need of the following:
4x57.1->66.56mm hub bore adapters, 5x112 bolt pattern
4x20mm 5x112 bolt pattern spacers
They'll need to be in this sandwich order:
Wheel -> Adapter -> Spacer -> Hub
When you're finished with this, your wheels will stick out just about 5/8 of an inch further than the stock wheels (-4.7mm + 20mm = +15.3mm)
Good luck, hope they look as good on the car as painted stockers, I got my fair share of compliments on the meteor black 18's that I used for my winter setup
O_E