Generally, it'd be most electricity-efficient (though possibly less "MTBF-efficient") to increase voltage and drop current. Unsure if this is significant with low voltages in DC components. However, it's now much more possible to do by increasing voltage and dropping "power tune" in the newer AMD cards.
You can't drop the current directly, a GPU operating at maximum hashing speed (not maximum clocks per se, as already pointed out) needs a certain voltage to be stable. What we can optimize is the lowest possible voltage at which it does that. When we drop the voltage, it reduces the leakage of the chip which then in turn leads to less power being drawn. In effect, the input current is reduced considerably but indirectly. Throttling the current via powertune will seriously impact the chip's performance so that's no good in our case.
I have my R9 290s hashing at 805 kh/s instead of 875 because the difference is 75 watts per card. Not worth it for me. 911/1250 clocks at 1.00V on the GPU draws 150W on my watercooled card and 170W on the aircooled ones. Nice and efficient. My 7970s are doing 700 kh each at around 150W in a cold room. These are DC input wattages based on the VRM input current.
I could make a few percent more money at today's rates but after electricity the difference really is almost negligible, and the cards are much happier running cooler.
