Don't have specific numbers, but my wow windows feel a lot more responsive with the change.
5x windows @ 2560x1600 across 2 screens and 2 graphics cards.
From being laggy on HH runs to awesome rift destruction now. And the Dalaran experience is the smoothest its ever been.
edit: running on: i7 940 @ 4GHz, GTX480(main) + GTX285(alts), 12GB DDR3 RAM and a 128GB SSD