Gigabyte R9 280x Rev2 is best. It produces 730k/s while its undervolted to 1.075V at 64C . Sapphires and other makes are power hungry. No other make can come close to Gigabyte version.
Did you flash the bios to unlock this card? I have like 8 of these and have no control over the voltage in Afterburner. Many of the newegg reviews also say the card is locked.