I've got some Sapphire 280X Dual-X cards as well, though I don't recall if they're from the 20G batch or 25...

In any event, as you're already on Windows, download GPU-Z and check the VRM1 temperature (it's at the very bottom of the graphs). This run of card has a throttles itself as soon as the VRM hits 110C, which it will do on stock settings, even if it's all alone in an open-air case...

My fix: Undervolt the stupid things to 1085mv, run at 900 mem clock, space them out at least three inches from each other, and push air over them via a box fan. I have NO idea how these cards are going to perform in the summer, but I'm fearful.

My steady KH per card: 730