I jsut put 20" tires and wheels on both my 98 A8 and my 01 A8L, I went with 255/35/zr20 on both cars and have had no problems at all with turning or rubbing. I also want to lower both with the 1.2" springs but haven't done it yet. You will notice the hydroplaneing a lot more with the wider tires, like others have said. If you choose to go with the larger rims don't increase your offset, decrease it if you must. But you will always have a better ride if you can stay with the stock recomindations. I hope this helped.

 

That's because you have an A8. The wheel wells dimensions are different than the smaller models. When you lower the car, you may have trouble with rubbing.