just use this and you will have between 700 and 750 kh/s

"no-submit-stale" : true,
"scrypt" : true,
"gpu-threads" : "2",
"gpu-Engine" : "1090",
"threads-concurrency" : "8192",
"gpu-fan": "80",
"gpu-memclock" : "1500",
"intensity" : "13",
"worksize" : "256",
"lookup-gap" : "2",

or for cgminer (easyer for me with trixx setting speed for mem and clock @ core 1090 and 1500 mem

cgminer --scrypt -o url:port -u username -p password  --thread-concurrency 8192 --worksize 256 -g 2 -I 13 --lookup-gap 2 --shaders 2048 -s 1 --expiry 1 --queue 0 --no-submit-stale