I was initially seeing some performance loss with an A8 + 6670 in dual graphics. I made a few changes though.
I connected the monitor through the onboard video out.
Pit stop in BIOS, forced the IGD, set it to 1gb framebuffer (matches the 6670), enabled surround view.
Installed newest chipset drivers, sans SATA driver.
Installed Catalyst 13.1.
Turned on, opened CCC, enabled Dual Graphics and checked the box for forcing dual graphics for applications with no profile.
Got huge gains in Doom 3 BFG edition, Blacklight:Retribution, WoW, Skyrim, Mass Effect trilogy and Dead Space games.
I get even better gains when I overclock the IGD from 760mhz to 800mhz to match the 6670 and force the XMP profile of my Ripjaws.
Haven't tried any others though.
MSI R6670 1gb GDDR5
BioStar Hi-Fi A85W
4x4gb Gskill Ripjaw X
Cooler Master s524 HSF