Hmm, thinking about this more, I think I've got the numbers the wrong way around in my previous post. Doh! Looking here:
http://www.raceandroad.com/offset.asp
and here:
http://www.audi-sport.net/vb/showthread.php?t=21785
So try again, you will be reducing the offset if the adaptors are 15mm to an equivalent ET of 21. Your rim is 228mm wide, so the face of the rim will be 93mm from the hub and back side of the wheel will be 135mm beyond the hub face into the car.
Standard S3 wheels are I think ET 33 (or maybe 35?), and are 7.5inches or 190.5mm wide. So using the same calculations, the centre line of the wheel is at 95.25mm, less the 33mm for the offset, giving 62.25mm projection from the face of the hub.
Based upon the AS.net post linked above, the general concensus is you can get away with 20mm spacers on the rear with normal S3 wheels and just avoid rubbing. However, if you have 18 inch rims or lowered, 15 or 10mm are the max you can go for.
So continuing to use the Avus wheel as an example on a non-lowered car, you can use 20mm spacers so that pushes the outer edge of the rim to a dimension of 82.25mm from the hub.
So on that basis, I think your proposed setup will be too wide by roughly 10mm. But I'm happy to be proven wrong.
Note that this is a rough and ready calculation and doesn't take any sort of negative camber, lowering or the 'bulging' effect of the tyre beyond the rim into account, and is purely based upon some 'back of fag packet' calculations based upon a guestimate of the hub adapter thickness and the offset of a standard Avus wheel.
