What kind of hashrate are you getting with the 280X with Kalroth's cgminer?
715 at stock clock rates. That's actually just about the same as with the unmodified cgminer since the 280x already likes 2 threads. I have my overclock script running now, which will take until tomorrow to find the best clock rate.
The 280x's (all Powercolor) settled out to 715 for GPU 0 (no idea why it's so finicky) and 740 for the rest, and that's undervolted to 1.1V. Unspectacular, but respectable.
Weirdly, the same core clock that gives 740kh on GPUs 1 to 4 drops GPU 0 down into the 500's. It insists on the stock 1030Mhz *shrug*
Getting 750KH/s undervolted to 1.081 with the core clock upped to 1080, It dislikes changing the memory speeds.