We will be working on a tune for the v10 S6, and my Tuner apparently has done S8s before. I would guess something will be available by the summer time.
Currently, the car has the engine out while we explore different options to increase performance. Once everything is back together, then the tuning begins. I will have a build thread up pretty soon on AudiRevolution.
I created the C6 v10 S6 Info Dump, to have a place to gather all the data on that model. However, if you have a v10 S8, then there is a lot of good info that will also apply to your car.http://audirevolution.net/forum/index.php?topic=2614.0
Feel free to PM me if you have any questions.