i use
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
cgminer --keccak --gpu-fan 85,85 --gpu-engine 1150,1150 --gpu-memclock 1000,1000 --gpu-powertune -20,-20 -I 14,14 -w 256,256
for 387khps per each r9 280x which is basically the same card as 7970
Why do you set PowerTune -20 along with a fixed and high GPU clock? It's completely illogical.
PowerTune will decrease your GPU clock on the fly to keep the TDP (including your -20 offset).
So... if you want to limit the power usage (to limit your power draw and/or to keep things lower/cooler), why don't you just set a slower GPU speed to start with...?
This is not a big deal for SHA-3, since only GPU speed matters. But with scrypt and other more complicated algos, the GPU:VRAM clock ratio is also important and PowerTune can easily sruw up that ratio (at random points in time, as it will constantly play with the GPU clock while keeping the VRAM clock constant -> there is an sgminer feature to keep them at a contant ratio with auto engine clock based on a target temperature which can be useful).
with my 7950 powertune is important, +20 need to be for proper hashrate, -20 drop hashrate and make gpu flapping
with 280x it do nothing, completely useless, no change in hashrate, stable gpu load