I don't think AMD have optimized their drivers for this game yet at all. I'm hoping for a bump in performance once new drivers come out. Crossfire support hasn't happened yet either; at least my rig sees no benefit.
Edit: After a little googling I found this from twitter:
@CatalystCreator: We're looking at Witcher 2 for single and CF - will release hotfix driver as soon as possible
@CatalystCreator: We always try to get CF support the day a game releases, but sometime we don't see the final build of a title until it releases...
Maybe since the game has nvidia branding amd didn't have the final build with which to work on drivers?