On Windows I get 4.1 Mh/s with each of my 2 R9 290 non X, on a test run more than 6 hours.
Thank you!

which driver and can you post your settings?
My settings:
"intensity" : "19,19",
"worksize" : "128",
"lookup-gap" : "2",
"gpu-threads" : "4",
"thread-concurrency" : "20400",
"shaders" : "2560",
"expiry" : "1",
"failover-switch-delay" : "60",
"gpu-dyninterval" : "7",
"queue" : "0",
"scan-time" : "1",
"kernel" : "x11mod"
clocks 1080/1300 GPU/Mem, Catalyst 14.4