I'm running 10 5830s across two power supplies and the system draws just over 1500 watts. Have a damn hard time achieving greater than 2 MH/W with these inefficient cards
If your MH/W starts to become a problem you can always start looking into undervolting the cards, i've gone on my 5xxx based mining setup from around 6,1 GH/s and 1,9ish MH/W last summer down to 4,7 GH/s and 3.1 MH/W without changing any equipment. Was pretty much forced into it by my high power costs (0,18$/KWh, silly sweden
then again it kept me warm during the winter! ). Most cards are running at 0.95V while i've got a few random cards that can only do 1,038V and 1,06V due to being non ref cards. On average the 58xx cards can handle around 700 Mhz at 0.95 and the 5770 ive got can do 750ish. Just goes to show what you can do with the 5xxx cards as long as they support undervolting.
Might also add that i'm running 80+ bronze PSUs atm so I could even increase my MH/W further. Long live 5xxx cards!