Originally Posted by TheLawIX
It's not necessary to disable it. Your cards will only sit in a low power state when you are on the desktop or not in need of the extra power. They WILL NOT revert to a lower power state in-game. Thus there will be no performance difference with it off.
not necessarily true
i've had two games (Dark Souls, Deus Ex: HR) refuse to kick in the second GPU from low power to full 3D clocks with ULPS enabled. a system restart fixed the issue, but it seems to happen from time to time
could be driver related (using the 13.8 betas that were just released), though I reinstalled them twice now.