A well-built 7970 rig is way more efficient that you listed:
2060 Mhash/sec @ 498 watts -- 4.13Mhash/watt (918 mV, 900Mhz/160Mhz) (all cards stable)
If you have efficiency concerns, you're better off with a modern FPGA, such as Icarus or X6500. They'll give you double the effeciency compared to BFL.