For those having low rates with 280x try this ---> +300Kh/s each R9 280x just put gpu engine to 1160 and gpu memclock to 1775 (+320kh/s for me)
ich have 3 r9 280x and getting 354khs ( scrypt-n ) with each
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
COLOR 0A
cgminer --scrypt -o stratum+tcp://panda.nitro.org:3338 -u login.worker -p pw-w 256 -I 13 -g 2 --lookup-gap 2 --gpu-engine 1000 --gpu-powertune -20 --gpu-memclock 1490 --thread-concurrency 8193 --temp-target 75 --temp-cutoff 85 --gpu-fan 95