Put some bubble wrap on your hand and wear a hockey mask. You'll be all set.
But in all seriousness I appreciate these tests. Performance-per-watt is, IMO, one of the most important things to look at when getting a GPU. The only thing that'd make this more interesting is performance-per-watt-per-dollar, where you basically find out what is the best valued and most efficient GPU possible, but on a website like this where old hardware is reviewed, that doesn't really work out.
They could group them in separate price ranges. Someone can look at the group within their budget easier to find the most efficient.