For some, hitting the highest frame rates is priority number one. Never mind the cost. Never mind the power bill.
That last line might as well be the motto of our experiment here today. Efficient, these configurations are not. And the more GPUs you add, the worse the situation becomes.
If you really had to choose the fastest setup most considerate of energy costs, you'd have to go with three of AMD's Barts GPUs.
Running at more than 100 W under two GeForce GTX 570s and serving up better performance; that certainly deserves praise.
We also have to point out the power use of two GeForce GTX 580s. Just let the bottom bar in the chart scare you.