cgminer is so efficient you will need to turn down your OC.  Try it, then notice that you STILL get better MHash/s.  I actually had to turn mine down on a 1.x.x version because of this, and had to turn it down again in 2.0.6, but I'm still getting higher MHash/s now than I was with the 1.x.x version before I had to turn it down.  Note that it may take a few trials to find your new sweet spot.