Mythbusters did a test with AC on/off windows open/closed and came out of it it with no difference in MPG, this was an older car, too. I never gave much merit to that theory anyway, and I've never noticed a difference in MPGs with AC on or off a whole tank. I attribute a 1mpg difference (if that) to differing conditions (however minimal they may be.)
Haven't really driven the focus without AC though since Florida is in a state of constant summer aside from 2/3 weeks out of the year at random (i.e. one in December, then the other 2 in March? weird.)
I'll have to take a spin with no AC and see if I can feel a difference in power.
2012 SGM SEL Sedan 303a w/o Nav
Job 1 built 6/2011