I was able to do some performance characterizations today in the lab. I could only go up to 1 amp of supply current, but what I got was that the efficiency was largely inversely proportional to the input voltage. That is, the higher the input voltage, the less efficient Pi Power gets. At 6 volts, the efficiency is in the mid 90s; at 9 volts, the high 80s; at 12 volts, the low 80s and at 15 volts the mid-to-high 70s. That appears to be a constant across the load conditions I was able to select.
The temperature of the MOSFET was also seemingly tied to the input voltage as well. So for best results, go for as low a voltage as you can. The downside of that is that the lower the input voltage, the higher the input current requirements. The sweet spot seems to be around 9 volts. If you have an ordinary R-Pi and are not trying to go nuts powering peripherals, you probably could do just fine with an 800 mA @ 9V supply. That will give you some headroom.
I power mine with a 10 watt 12 volt supply, and that works well too (it draws around 300 mA).
The actual boards that are going to go in the store likely will not have the thermal issues the prototype did. I have high hopes that the full 2A draw will be available with the upgraded MOSFET and thermal design.