This probably sounds ridiculous, but I recently upgraded my 6-monitor setup to an 8-monitor one. The "primary" 6 monitors are running on an ASUS Radeon HD 7970. To add the additional two "occasional use" monitors I added a cheaper Sapphire Radeon HD 6450. They're both running the AMD Catalyst Win 8 Preview Drivers.
Dragging and dropping between screens went from buttery-smooth (60 fps+) to what I'm guessing must be around 30 fps. This is between screens on either the same video card or between cards (no difference). Launching the Windows 8 start screen is substantially slower (instead of instant it takes a few seconds with very choppy animation), even though it only appears on a single screen running on the 7970.
So I don't really understand why but I guess the 6450 is holding the 7970 back somehow - Windows is only running these animations as fast as the slowest card? Is that correct?
I can live with the slightly slower drag-and-drop but the start screen is very annoying. Can I expect this situation improve when updated drivers are released? Or do I have to just cough up the cash and buy another Radeon 7000 series to replace the 6450? The cheapest 7000s run around $200 so if I can find a better solution I'd rather not.