This S8 wheel should be a +46 offset. 275/35/20 will be too tall and may rub front and rear. This is the recommended size for an A8

255/30/20 will fit. You can also use 275/30/20 tires, these have a higher load rating and will fit fine given the wheel's offset.
I've had 275/30/20 tires on 20x9 et40 wheels on my 2006 A6.
I currently have 275/30/20 tires on 20x9.5 et44 wheels on my 2007 S6.