Optimized settings for Sapphire 290x with Elpida Ram and stock BIOS (18,9Mhash/s):
"kernel": "groestlcoin",
"gpu-engine" : "1000",
"gpu-memclock" : "1500",
"gpu-powertune" : "30",
"xintensity" : "300",
"vectors" : "1",
"worksize" : "512",
"shaders" : "2816",
"thread-concurrency" : "27400",
"difficulty-multiplier" : "0.0039062500"