Impedance has little to do with efficiency. Lower impedance will make headphones easier to be driven with lower voltage source (portable source) because output wattage equal voltage times voltage divide by resistance. Efficiency means the amount of wattage that's converted into sound pressure by the headphone driver, so you can have a driver that has very low impedance, but only converts very little of that wattage into sound thus it has low efficiency. Drivers that has low impedance but also low efficiency are you typical speaker drivers.
Headphone spec usually has it listed by sensitivity. xxxdB sensitivity usually means at normal listening position the headphone will output xxxdB of 1KHz sound pressure if input power equals to 1mW.
As for 812.... it still looks like 701 to me only much better built... (too much bad memories from AKG's attempt to bank on 701 spin-offs..) I personally like the approach that Sennheiser took with HD800: scrap everything and design it new from ground up. That's really the way AKG should do if they still intend to get back into high-end market, especially when their past reputation in high-end full-size headphone has mostly gone down the drain after k1000 and k701.
Just my 2 cent.
Edited by e19650826 - 10/4/13 at 1:46pm