Hmm, that makes sense that Powertune is the culprit. Has anyone figured out how to disable or bypass Powertune?
As I mentioned earlier, the total 'official' power available is about 150W (75W from PCIe and 75W from 6-pin). I'm sure this solution can provide more power, but I wouldn't go TOO much higher for reliability's sake, personally.
It would also be nice to be able to re-flash the card to a 7870 and run at a reduced clock rate, but it's too early to tell if this is possible. It seems the hardware is there on the chip (same # of transistors) but somehow disabled.