Yeah, I'm familiar with the S4 and S5, and I've seen a couple S5s around here. They've been available in the US since late 2008/2009. I got to test drive an A5 coupe, but never an S5. I was really close to getting the A5, but then I test drove a BMW 335i coupe and I was sold. It made me realize how slow the A5 was. The S5 and 335is are awesome, but were a few grand over what I had decided I would spend on a car.
I'm not sure about parts and such, but I had thought Audi and VW shared certain engines (e.g. the A4 and the Jetta).