Update:

I must be a linux noob.  I tried windows on a whim and now I get stead a steady 745 kH/s.  I've seen it go higher but I don't dare push it considering one of my three cards is bordering 90 deg C.  I'll tinker some more when I get a better cooling solution. 

Windows 8.1 x64
Catalyst 13.11 beta 9.5
SDK 2.7

-I 13 -g 2 --gpu-powertune 20 --gpu-engine 1070 --gpu-memclock 1500 --thread-concurrency 8192

A couple observations:

For some reason windows only needs one dummy plug for all three cards and the cards hash about 10 kH/s faster without them. 

This has been a great learning experience. 

Thank you for all your input

Also,  as an update I am now able to get peak hashes of about 710 kH/s.  The hashrate is pretty inconsistent though and will jump around between 703 and 640 while momentarily peaking at ~710 or dipping to ~620.  Overall this leaves me with an average of about ~650 kH/s


-g 2 -w 256 -I 20 --lookup-gap 2 --thread-concurrency 22270 --gpu-engine 1045

I've tried several settings for thread concurrency the highest I can get it to go is about 22272. 

Could you post your whole config? 

Thanks

I tried all the above settings and only reached a max of 580kH/s.  This card seems to be unlike other r9 280x's.  Today I had a little more time to play around and ended up with the best (for me) results using:

./cgminer --scrypt -I 20 --gpu-engine 1050 --gpu-memclock 1500

This gives me 660kH/s

I would like to get up to 700kH/s but I'm out of ideas because all the other tuning guides for 7970 and 280x seem to decrease my performance substantially.  Changing memory speeds at all decreases performance so I'm not sure what else I can do. 

Any ideas?

**I would rather post this in a more relevant area, but apparently this is the only area i'm allowed to post in**

I followed this guide to build my first scrypt mining rig:

http://www.cryptobadger.com/2013/04/build-a-litecoin-mining-rig-linux/

I'm currently getting only about 550 kH/s per card (for scrypt mining) under the following conditions.

cgminer 3.7.2

./cgminer --scrypt -I 13 -g 2 -w 256 --shaders 2048 --gpu-powertune 20 --thread-concurrency 8192 -o stratum+tcp://xxxxxxx -u xxxxxxxx --no-submit-stale

I know a substantial increase is possible because a member here, glitchboy, reported getting 750 kH/s with the same card:

https://bitcointalk.org/index.php?topic=343226.msg3683234#msg3683234

Could anyone here help me figure out what I'm missing?