Make sure you ran driver sweeper and did a clean install. There could be some lingering issues from the previous drivers. Also, what other games are you playing? Have you run 3dMark Vantage and 2k6 yet just as a general test? Running just one 5770 in my Crossfire system gets around 13,600 points in 2k6 on default, compared to the 4830 in my sons system which only gets around 10,300 (2k6 gets pretty cpu limited for me around the 13,000 mark, running in Crossfire only gets me around 15,000).
Also, Need for Speed Hot Pursuit is very poorly optimized, so I wouldn't put too much responsibility onto that title as far as gauging performance goes. It hits major frame drops in some races compared to others, and in general just doesn't perform very well on any system compared to other racers. Try benching Dirt 2, Alien versus Predator, Metro 2033, Black Ops, etc with Fraps and compare the average and minimum frames per card and see what you come up with.
Edit:
Here is a link to a guys results comparing his 5770 performance to his 4850 performance. He is running a Core 2 Duo also, so this may give you a better idea of the kind of gains you should be seeing.
http://www.rage3d.com/Board/showthread.php?t=33953894