i had a crossfire set of 7850's and switched to a singlew card 7850 cause i only noticed a +10 in fps this was awhile back however and i wondered with the new drivers if it made a drastic improvment or not?
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.
The latest driver fixed the micro stutter issue that crossfire was experiencing but if you only saw a 10fps gain with the additional card it's probably because the game you were playing wasn't made to take advantage of two cards. It's worth a shot though to see if it's been improved.