Sorry I was assuming you meant the 18" Avus
Calculations show the 9x20s will sit 7mm further out than the Avus, and given how tight mine are I doubt there is that much room with 20mm. You'd need 13mm spacers to keep them in the same place as mine so I reckon you're spot on with 10mm front and 15mm rear.
I can try mounting one of my FE 20s on my 20mm spacers tomorrow, and see where they sit. The extra 1/2" of width on the D3 20s won't make much difference on tyre clearance.
Looking at ebay you can get complete sets of longer bolts including lockers, as long as the radius seat is actually the right one for VAG wheels. A lot of the time people try to pass off Porsche radius fitment as the same but it isn't, so something to watch out for. I had a hell of a time finding proper radius nuts for the racecar but the situation may be different with bolts.
Edit - duh reading fail! only just spotted you said rears only
I'll try front and rear just for info.