I just put new tires on the S8 yesterday....275/35/20's....Falken Azenis PT-722
Discounttiredirect.com had a sale on ebay two weeks ago, $100 off, I got four of them from the local discount tire, mounting, balancing, disposal, tax and road hazard (replace the tire for something like 3 years after purchase) for $920....that was a deal! I know the manager (audi guy) and he matches any deal.
Obviously they are only a day old....but man is the car smooth again on 20" wheels. My old summer performance tires were only 15K miles old....but were shot. Never again. Just need A/S's for this car, plus I run snows in the winter.