Dead Cat,
I have been doing a bit of investigating and found the problem, IE9 coupled with the latest Nvidia drivers (285.62) includes a GPU accelerator function that reserves a certain amount of rending power just for IE9 tasks that never gets released to other processes. I believe the beta drivers also included this.While IE9 is open, for example Battlelog, single GPU performance is hit by up to 20% while in-game. SLI performance is more pronounced and gets hit by up to 50%. The impact is not immediate, it can take up to 2 hours (oddly) to take affect after a clean restart of Windows. This feature is on by default in IE9: Internet Options > Advanced > Accelerated Graphics. To disable, just tick 'Use Software rending instead of GPU rendering'.
Yet another reason why Battlelog is a big pile of steaming shit. IE is only doing what it is intended to do, web browsers were never meant to be a launch platform for a game and the window in which the game is played is effectively a browser window. Hence the problems.
Since turning this off I am getting upwards of 70fps on the smaller maps with everything set at Ultra and around about 50fps on the larger maps. But most importantly it plays like that!
Sadly it hasn't improved my game play any