These are the performance per Watt-hour results for PCMark Vantage. The differences are small, but the overclocked setting at 3.6 GHz clock speed provides the best ratio between high performance and power consumption. It is interesting to see that the 3.0 GHz Phenom II X4 940 offers better power efficiency than the 2.8 GHz Phenom II X4 920 (2.8 GHz).
We put the results into a more visual chart. You can clearly see that the PCMark Vantage score per Watt-hour keeps increasing up to the 3.6 GHz setting, but drops considerably at 3.8 GHz. This means that 3.8 GHz do not make sense anymore from a power efficiency standpoint.