I've got a HIS r9 270X IceQ²-whatever. Getting 470-480 kH/s without any errors with these setting:

setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
cgminer.exe --scrypt -o stratum+tcp://eu.middlecoin.com:3333 -u xxx -p xxx -w 256 -I 19 -g 1 --thread-concurrency 15232 --temp-target 70 --gpu-engine 1120 --gpu-memclock 1500 --lookup-gap 2

Running at 66°C, Fan 2150 RPM