If that worked for you and both match hashrate now... try my 7950 bat file...
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
cgminer.exe --scrypt -o stratum+tcp://pool:port -u xxxx -p xxxx -I 20 --thread-concurrency 21712 -g 1 --gpu-engine 1050 --gpu-memclock 1500 --gpu-powertune 20 --gpu-fan 90 --temp-cutoff 85 --scan-time 1 --queue 4 --no-submit-stale
I changed my values to match what you have above for your clocks.
640 and 600 kh/s ty man!
That makes me wonder where you got your 280X... it behaves like an old Tahiti chip and not the newer Tahiti XL2 or XLT in these recent R9s. You sure someone didn't sell you a 7950 in a 280X box?
What temps you got now... if you aren't at 75 C yet then try changing the clock to 1100 and the mem up to 1700. If it takes those and can hang around 75-78 C, then I think you may somehow have gotten a 7950 and NOT an R9 280X...
Here is my EXACT bat file for my 7950 @ 660+ KH/s... I CAN and HAVE pushed it to 700KH/s with 1200 clock 1800 mem 1200 vddc and +20 powertune but i dont push it like that any more now that it is a year old.
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
cgminer.exe -d 0 --scrypt -o stratum+tcp://fry.minebig.com:3341 -u xxx -p xxx -I 20 --thread-concurrency 21712 -g 1 --gpu-engine 1125 --gpu-memclock 1800 --gpu-vddc 1150 --gpu-fan 90 --temp-cutoff 85 --scan-time 1 --queue 4 --no-submit-stale