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.