For the hell of it, I'd take it back and get another of the same card, just to be sure it wasn't a fluke
Is there something in the driver itself that is switched on that could be influencing the framerate?
I installed it from scratch so it's default + I checked all the settings and they seem fine. I'd still be looking at the PSU, tbh.
Don't forget I'm still getting >1000FPS in OpenGL it's just a directX problem, so I don't think there's a problem with the PSU, it's not logical that it would only affect DX.