I run quite a few Sapphire DualX and Sapphire VaporX 280X cards.  Undervolted to 1.000v or 1.050v @ 900core/1250mem get 270/sols.  This normally results in 125watt-140watt draw at the wall per card on a Gold rated power supply. 

I also have quite a few Sapphire 270X.  At 1.000v or 1.050v those run at 75watts at 900core / 1250mem at 170/sols