I'm not sure how relevant that is with regards to performance per watt, but I do know that heat is one of the most expensive (by-)products of electricity.
AMD's chips aren't egregiously power-hungry, they just sell into a market where the product segments are defined by the thermal and electrical limitations of the PCIe expansion card form factor. It will always be the case that the top of the line card draws as much power as can be delivered through the slot and two extra connectors, and as much as can be dissipated through a two-slot cooling system. If AMD can safely operate their chips at a higher temperature, then their cooling system will be a bit more efficient, so their thermal envelope expands a bit and they can increase performance by a few percent.
Early indications are that NVidia's upcoming Maxwell architecture achieves significant efficiency improvements due to being designed with mobile use in mind, but it hasn't yet been demonstrated in a large enough configuration to compare against high-end desktop graphics chips.
AFAIK the only reasons devices aren't run at arbitrarily high temperatures is that material properties change unfavorably, or you risk fires.
Or, approaching things differently, if you're only going to keep your graphics card for two or three years, there's no reason to let the fans spin up to an audible level until the chip reaches at least 80 degrees C.
Addressing a common misconception: fan speed/noise is not directly related to how much the graphics card is heating up the room by. Whether you run the fans at full tilt or you let the chip almost melt, the heat output will depend only on the workload.