I'm using these exact cards and getting 28.5-29 mhs in claymore, pulling 100-110 watts from the wall each.
That is very power efficient. I believe.
Yeah. I've found that lowering cclock as much as possible helps you undervolt a little more and draw less power. I'm really happy with the efficiency.
Edit: I'm also using titanium PSUs operating at 40-50 percent of capacity, which helps.