I am having almost EXACTLY this problem.
My system takes an additional 10 or so seconds to post compared to my old gtx 580 sli, and games with vsync on are incredibly laggy & unplayable, framerates are in the 10-15 range often even with settings set to lowest. Audio crackles like mad too.
If I disable vsync, framerates improve greatly HOWEVER there is definite stuttering, or microstuttering. It is not smooth AT ALL.
If I run windowed/borderless, it's smooth as butter however crossfire is being disabled. If I run one card or turn off crossfire, it's smooth as butter. Again though I'm losing a card...
I've tried disabling ULPS, set amd overdrive to 50% power limit and 100% fan, turned off frame pacing, switching sound cards / using onboard sound, fresh install of windows 8, nothing made a difference.
Asus P6T Deluxe (x58)
i7 920 @ 4ghz watercooled
3 x 8gb 1600mhz
Sapphire 290X crossfire
Corsair HX1000 PSU
OCZ 128gb SSD