https://bugzilla.kernel.org/show_bug.cgi?id=57281
--- Comment #7 from Michel Dänzer michel@daenzer.net 2013-04-30 13:32:46 --- Does not enabling Option "BackingStore" in xorg.conf help?
The high power usage might be due to the discrete GPU being powered on, but the X server isn't using it, so it doesn't help performance.