Yep did A5 seats in my old A3, it's not a straight fit but it can be done... Also you could fit the rs4 buckets if you wanted?
The new S3 seats fit, in fact any seat across the range should fit in the fronts but you may not get all the functions working
I have a friend who is looking to sell his black electric and heated leather seats from an A3 sport back. Pretty much direct swap apart from some wiring changes depending what's in the car already
I wil have to find out how much he wants for it.
All he mentioned was that he needs an original set of seats to replace the leathers that he wants to sell.
The seats are electric, heated A3 full leather seats.
I can add the wiring under the seats for you... the wiring itself costs £70 and the installation costs £150.