From a MH/W basis it is hard to beat 5970.
My rigs pull about 250W per card (memory underclocked to 160Mhz).
I run 3x5970 per rig.
Using usb key for OS, underclocked sempron, and MB w/ everything unecessary turned off the entire system pulls ~860W at the wall.
Rigs get ~2.2GH to 2.3GH on 860W or ~2.5 to 2.7 MH/W.
I don't think it is possible to do much better than that unless you go FPGA or wait for 7800 series cards.
Do you think the efficiency would hold using 5870's? I imagine it would be similar since the GPU's are identical.....
It likely is very similar. The 5970 likely is slightly ahead because there is some de-duplication of minor components. Also to avoid thermal overload it is my understanding that AMD takes chips from the top(most efficient) bin when building 5970s.
Still although I don't have any specific stats I would figure a rig based on 5870s should get pretty close to 2.5 MH/W.