Crossfire performance is pretty much exactly how you describe for Sli, although I have personally found that the frame rate drops are more severe than a single card by around 5fps. For example, at busy Atherytes it drops to around 33 fps for me on a single card (Radeon HD 7870 1100hz edition running maximum settings at 1080p), but floats around 27-28 with crossfire enabled. When the fps is high, its very high, however like you I rarely see both cards running at more than 60% utilisation.

I have also experienced behaviour similar to a 'memory leak' with crossfire enabled - the performance gets worse over time until the game is restarted. This may be the game not correctly recognising the ram as the benchmark shows me as having 4gb, yet with how crossfire works only the total ram of 1 card (2gb) should actually be considered.

Right now it just appears to be poor driver optimisation at fault as fps does not appear to be directly related to what is going on on-screen. Additionally, I have found that changing display resolution has no bearing on the framerate which is odd - almost bottleneck behaviour, but running an i5 ivy o/c at 4.4ghz that's very unlikely.

Fingers crossed we see improvements from both Nvidia and Ati over the next few weeks - as Square Enix have been discussing patching in dx11 and 4x resolutions, they are going to need good drivers as there isn't a card on the market that wouldn't weep under the strain.