off topic, but what did you do to get 850 khs out of a 280x? I've got two (sapphire and a gigabyte) and under win 7 x64 i can only get 750Khps out of both ( total around 1500Khs. here's my config:
First off, set your lookup-gap to 2. Then, try stepping your intensity up to 16 or 17 - not all GPUs max out at intensity 13. Also try a higher thread concurrency, like 32765. Also also, try not setting the shader count at all, as it sometimes turns out better when cgminer determines the optimal value itself. And finally, also also also, see what happens if you don't set gpu-dyninterval at all.
Give this simple line a try and let it run a minute or two: cgminer.exe --scrypt -o <host> -u <user> -p <pass> --intensity 16 --lookup-gap 2 -w 256 --thread-concurrency 32765
See what happens. You can also try two rendering threads, by adding -g 2, to see if that changes anything.
edit: Scratch all of that. My bad - for some reason I kept reading 290X instead of 280X. My apologies.
