If this helps anyone, here's the config file I use to get 6.8 MHS out of 3 280x GPUs:
.\sgminer -k darkcoin -o stratum+tcp://stratum.multipoolmining.com:5555 -u my wallet address -p x --lookup-gap 2 --thread-concurrency 8192 -g 2 -I 13 --auto-fan --gpu-fan 0-85 --temp-cutoff 90 --temp-overheat 85 --temp-target 70 --gpu-memclock 1500 --gpu-engine 1180 --gpu-powertune +1
I played with many configs to come up with this one.
I get the similar hash rate with 4 x 280x. But mine are Gigabyte and are notorious for hahsing less than other brands. I get 1.6 average each.
Try this: thread-concurrency 8192 -I 13 --lookup-gap 2 --worksize 256 -g 2 --gpu-engine 1160 --gpu-memclock 1450
running 9.6 MH/s with 4x Sapphire r9 280x Vapor-x