On my Sapphire DUAL-X R9 280X's I get ~360Kh/s per card with these settings:
-I 13 -w 256 -g 2 --lookup-gap 2 --gpu-powertune -20 --gpu-engine 1000 --gpu-memclock 1500
Two same cards, almost the same settings -
360Kh/s"gpu-threads" : "2",
"gpu-memclock" : "1490",
"gpu-powertune" : "-20",
"intensity" : "13",
"worksize" : "256",
"lookup-gap" : "2",
"thread-concurrency" : "8193"
Sapphire R9 290 Tri-X -
458Kh/s"gpu-threads" : "1",
"gpu-engine" : "1000",
"gpu-memclock" : "1490",
"gpu-powertune" : "10",
"intensity" : "20",
"worksize" : "512",
"lookup-gap" : "2",
"thread-concurrency" : "27000"