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.
You need to bios mod it.
Easy, follow crypto badge's guide. (google it)