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"
Nope it's not happening, I can't get the cards to run over 150Mhz mem clock unless I use sgminer 5, which isn't giving me a better hash rate, on bet WU.
I'm going to update the drivers and recompile the miner source.
Also, which miner are you using? If it's the one in the OP, the difficulty-multiplier, TC, and shaders (i think) have been taken out of the code, because it was only useful for scrypt.... According to the thread anyway.