I am using this for my 280x, getting 2.2mhs
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
sgminer.exe --algorithm x17 -o stratum+tcp://people.suprnova.cc:3674 -u -p --intensity 18 -g 2 -w 256 --lookup-gap 2 --thread-concurrency 8192 --gpu-platform 1