^Yeah, I'm not sure on the specifics but 8x CSAA does everything 4x MSAA does with a little additional effect. You won't get better performance out of CSAA. I don't get much of a performance hit between forced 4x MSAA and forced 4x EQAA (AMD's counterpart to 8x CSAA) in the games I use it for (mostly low-res texture console ports though).
If you're looking for an AA solution that has better performance than MSAA, try a post-process solution like FXAA or MLAA. Be warned that such AA methods also come with lower quality and a little bit of blurring.
This article over at Tom's Hardware is a good overview of the different types of anti-aliasing.