thx to your help

If you have driver ati 14.6 beta, you can try this config
I have now 2.7Mh/s average with R9 280X MSI Gaming
{
"name": "x15",
"nfactor" : "10",
"algorithm" : "bitblock",
"intensity" : "19",
"worksize" : "64",
"gpu-memclock" : "1250",
"gpu-engine" : "1100"
},