to be honest you don't "need" to run any, but.......they look like 4 spares. 20mm spacers are prob a little much esp for the rear (I say that because the front is flared). 20mm are a little big in my book for a stock car. 10's would be more suitable, don't forget to see if you need longer bolts too, you should bet about 7 or more turns, if not you'll need longer
On a Quattro you really need the same rolling radius otherwise you'll destroy the centre diff. Vortex will tell you 3% difference is ok - 255/40 is 2.5% bigger than 235/40. Personally, I wouldn't risk it.
Technically you could fit 255s on an 8" rim - the C5 S6 does stock - but they'll bulge a little.