Well, no. Voltage is only one part of power usage, higher clocked cards use more current than lower clocked cards at the same voltage. That's why overclocked cards get hotter even when not overvolted.
Yes, power consumption scales linearly with clockspeed. But performance also scales linearly with clockspeed.
Higher performance at equal MH/W for the cards, means higher efficiency for the overall rig since CPU, MB, RAM, etc remain the same.
Power consumption scales quadratically with voltage. Clockspeed -and thus performance- does not. Therefore undervolting almost always gives better MH/W, but overclocking is completely sane from an efficiency POV.
Performance does not scale linearly with clock speed at all. (try 500mhz and 800mhz and you will see)