Hi all, I was, like some unlucky others here on this forum, struggling to get my R9 280X running above 300 Kh/s. I've tried the usual recommended I 13 and TC 8192 but no matter what else I tinkered with, it simply did not go above 300 Kh/s. First I thought it was coz I had 3 different GPUS. Then maybe coz I had only 2Gb RAM. I upped that to a total of 6Gb Ram and that made a little difference. But the real differences were in overclocking my R9 and setting a temp target. The latter coz I noticed each time the card reached 300 it suddenly dropped to like 250-260 Kh/s along with the temp dropping from 80 degrees to 76 degrees, as if there was a temp limit that maxed my hash rate too.

The overclock tip I got from this cgminer manual, which gives exellent tweaking pointers! https://raw.github.com/ckolivas/cgminer/v3.7.2/SCRYPT-README
A quote:
"Then start overclocking the eyeballs off your memory, as 7970s [edit: R9 280x is a rebranded 7970]  are exquisitely sensitive to memory speed and amazingly overclockable but please make sure it
keeps adequately cooled"

Here are the details:

Stable at 780 Kh/s on average  
Club 3D 7850 172 Kh/s
Club 3D 7950 310 Kh/s
MSI R9 280X 290-320 Kh/s

export DISPLAY=:0
export GPU_MAX_ALLOC_PERCENT=200
export GPU_USE_SYNC_OBJECTS=1
cd /home/c/vertminer-gpu
./vertminer --nscrypt -I 17,20,13 --thread-concurrency 8192,21712,8192 -o stratum+tcp://vtc.poolz.net:3333 -u xxxxx -p x --gpu-engine 1000,1070,1200 --gpu-memclock 1250,1500,1800 --shaders 1024,1792,2048 -w 256,256,256 --temp-target 66,84,83

So far, I've not succeeded in getting it at like 350 Kh/s, perhaps someday. Also, I'm not sure if overclocking is good for the R9 in the long run. But hey, I'm happy now, and hopefully this post is helpful to others!