yes the improvement is drastic from what I have heard.
Previously, Crossfire had an issue which caused a lot of frame stutters as it was called. Frames were only being partially complete and thrown on screen and stuff like that to get it done as quickly as possible. his reduced the quality of the images and gave lower performance than it should.
AMD recently found a solution to this problem and has been working to implement it. I have hard that they released the fix in the 13.8 version of Catalyst. If so, you may or may not notice increased fps, but you should noticed increased quality and performance.