Can you explain what differences you experienced with "power efficiency" on? I imagine your card won't stay in the highest state then all the time? I picked up a 480 myself and hadn't really time to really get into it. I undervolted it by 25mV on the highest state but did not see such a noticeable drop in temperature. I will check it out this weekend.
So far I'm running with 1400 Mhz core clock, 2100 Mhz memory clock on a -25mV (1.125 V) undervolt on the highest state. With power limit set to +50% and a temperature goal of 75 °C I never leave the highest state under load anyway. Which is kind of confusing though, as some youtube videos suggested raising the temperature goal and power limit to achieve a better stability on the highest state (reduce thermal throttling). But my card reaches 75 °C (adjusted fan curve as I'm gaming with headphones anyway and it's still a lot quieter than my old 580) still I don't see thermal throttling.