Is this a consistent load over the entirety of the WU? Like, when you're watching the load of the card in Afterburner or GPU-Z or something, it never gets above 72%? Or does it dip to that low of load at some points during the WU? If it's the second one, that's common - the irritating inefficiencies in how DNETC has coded their ATI app. If it's the first, then I'm not sure...

If it is the first, I might suggest turning on Crossfire and seeing if that helps. And install the latest "application profiles" when you do Crossfire.