I use the following on my R9 280x's. Linux (Ubuntu 12.04 x86_64) with 13.11 Beta and cgminer 3.3.1:
Threads = 2
Powertune = 20
Intensity = 13
Core Speed =
Mem Speed = 1500
TC = 8192 (20480 has also worked for some of them)
worksize = 256
lookup gap = 2
You'll notice that I left the Core speed blank. Try starting at the factory-clock speed, and moving up in increments of 5 or 10. This, of course, will vary based upon the specific model.
My Sapphires have a 950 base speed.
My Gigabyte models have it set to 1000.
A powercolor one I googled had 850.
Ironically, setting the core speed to $BASE + 25 seems to be the magic formula for me. I run with 975 on my sapphires (getting 690, might be able to push it more with a larger PSU). My GB models get 710kh at 1025 core.
As they say, each card is different. It's all about tweaking.