With these settings, I have 900 K GPU
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
yacminer --scrypt-chacha -R 2560 -g 1 --buffer-size 2560 --lookup-gap 8 --worksize 256 --gpu-engine 1020 --gpu-memclock 1500 --temp-cutoff 95 --gpu-fan 0-100 --auto-gpu --temp-target 85 --temp-overheat 95
3GB memory GPU
Saphhire 7970
I actually found at NF=15 that LG=6 and LG=7 yielded faster hash rates than LG=8 for my cards. I would try settings for LG=6 through LG=10 (lowering or raising R to compensate) and see what yields the fastest hash rate for your card.
LG R
6 1883
7 2214
8 2560
9 2730
10 3091