Perhaps the higher voltage causes the mining speed to decrease (doesn't make sense, but neither does anything else that's happened with them). Since I only am running two until my risers get here, maybe I will just exchange the other two for vanilla vapor-x cards. If I can get those doing that they are supposed to, it would be worth the hassle for an extra ~150kh per card.
My limited understanding is that the cards have a feature called Powertune Boost which dynamically manages clockspeeds and voltages for maximum performance.
That may be desirable in a gaming scenario where the rendering load changes from scene to scene, but not so desirable in a constant 100% load scenario.
See:
http://www.pcper.com/reviews/Graphics-Cards/AMD-Radeon-HD-7950-3GB-PowerTune-Boost-ReviewMy Sapphire 7950s have a little blue buttons on them that change the BIOSes and apparently also enable/disable Boost. I had the buttons enabled during the first few hours of mining/testing (thinking boost = more performance? yes please!) and noticed that the clock speeds jumped from 925/1400 to 850/1250 all the time. Just like the picture of Metro in the review above. (which is mislabeled with FPS instead of Mhz on the y-axis but you get the idea)
Once I disabled the buttons and switched back to the regular non-boost BIOS, my clock speeds and voltages smoothed out and the hashing performance improved dramatically.
As Ive mentioned, I suppose the feature makes sense for games where you have rendering performance valleys and spikes that allow Boost to make the most out of the available TDP envelope, but it doesnt make much sense under constant load.