I knew that raising the voltage makes your consumption go up exponentially, but this seems a lot. Have you measured your consumption at default speed/voltage? If i'm not mistaken it should be 256 with 900mV, but don't take my word on that.
Just did a quick test on my 4 boards:
256 Mhz, stock mV = ~160Gh/sec @ 235w (1.4w/Gh)
278 Mhz, 1380 mV = ~200Gh/sec @ 383w (1.9w/Gh)
Measured at the wall using a kil-o-watt. Power supply is Corsair AX860.
I know the firmware I am using is converting those clock and voltage numbers (as it thinks it is talking to an avalon board) but the overall Gh and power numbers are accurate.