Power efficient?.. Approximately 1W/Gh as I see...
Let's compare with AntMiner S4. A set of 7 of your 300Gh sets (to achieve 2.1 Th) costs 1470 EUR and does not include PSU(s) and fans. Let's add to this price a shipping cost... It is curious, but looking not so profitable comparing to S4. At best it looks like S4 in terms of profitability, but 1W/Gh...... Too much.
Look at the test from July 4th: 150 up to 190 GHs at 116W = 0.7733 to 0.6105. Avarage that = 0.69W/GH. The nice thing is you can "play" with power efficiency when you have a adjustable powersupply. As long as input is between 11V and 13V the miner is stable (with proper cooling): you can play with powerdraw and hashpower.
And you are right about the S4 and S3 and other miners, they are nice too and I am not saying it is crap of unefficient. I personally also bought, tested and mined with Bitmaintech, KNC, Bitmain, Zeus and other miners, they all work and all have their (dis)advantages but such a stable, efficient and low noise miner as the Yiazo-YBF is I and other who have bought the Yiazo YBF, have never seen and expericiened before.
(sorry about my crappy English sometimes
)