System did 2630 mhash/sec in the 1.175 volt setting and pulled 1460 watts at the wall ... actually had to step it back because I kept hitting 1500 watts. Plus this was a 1200 watt PSU and although it performed this way for a month I've since decided the extra power wasn't worth it.
So at 1.12 volts and -125 memdiff the system ran 2520 mhash/sec @ 1170 watts (95 watts system) for 356 watts per card.
Flashing the 1.175 volt bios with 1.12 volt a 150 mhz memclock reduced the power by 150 watts for 1020 watts or 306 watts per card.
Are you running your memclocks at 150MHz or core clock minus 150? Anything less than 300Mhz (at least on the 58xx series) definitely impairs performance.