What settings are you using on X11 on your card, My MSI 280x are only achieving 2.1MH
Not a huge jump, but I'm getting 2.35 MHs on my 280x with these settings,
"xintensity" : "4",
"lookup-gap" : "2",
"gpu-threads" : "2",
"thread-concurrency" : "0",
"shaders" : "0",
"gpu-engine" : "1150",
"gpu-memclock" : "1600"