Have you tried lowering the VR temperature? It seems to improve the regulator's efficiency, brings the power usage down too
You convinced me:
My old overclocked situation:
615 Mhz /1235 mV -> 5.1 Th/s at 158 watt AC input.
Now:
557 Mhz /1150 mV -> 4.39 Th/s at 112 watt AC input
for 16.4% more hash power, I need 41% more power