It's been a long time since I had my c5 rs6, but iirc I think the s6 gained around 35bhp, main advantages were better fuel economy, faster throttle response and if you have gearbox remapped, you can drop into 5th @30mph giving tick over running so minimal petrol consumption.
You wont gain massive power increases, it's non turbo so around 5-7%.
If your still looking to do this, I would recommend Doug @ MRC to have a chat with.