You want to have as much system ram as you have in Vram. Here is what my conf file looks like, I'm getting around 820Kh/s with these settings with a sapphire R9 290.
{
"pools" : [
{
"name" : "xxx",
"url" : "xxx",
"user" : "xx",
"pass" : "xx"
}
]
,
"intensity" : "20",
"vectors" : "1",
"worksize" : "512",
"kernel" : "scrypt",
"lookup-gap" : "2",
"thread-concurrency" : "32764",
"shaders" : "0",
"gpu-engine" : "800-964",
"gpu-fan" : "0-67",
"auto-fan" : true,
"auto-gpu" : true,
"gpu-memclock" : "0",
"gpu-memdiff" : "0",
"gpu-powertune" : "0",
"gpu-vddc" : "0.000",
"temp-cutoff" : "97",
"temp-overheat" : "96",
"temp-target" : "85",
"api-mcast-port" : "4028",
"api-port" : "4028",
"expiry" : "120",
"failover-only" : true,
"gpu-dyninterval" : "7",
"gpu-platform" : "0",
"gpu-threads" : "1",
"hotplug" : "5",
"log" : "5",
"no-pool-disable" : true,
"queue" : "1",
"scan-time" : "60",
"scrypt" : true,
"temp-hysteresis" : "3",
"shares" : "0",
"kernel-path" : "/usr/local/bin"
}