The miner has been developed on omega drivers (14.12) but it is reported to be working fine and as fast on newer ones - BUT - some users have reported half hashrate on the first card and fixed it by installing catalyst version 14.9.
Thank you for solution.
I put amdocl.dll file from catalyst 14.9 (C:\AMD\AMD-Catalyst-14-9-win7-win8.1-64Bit-dd-ccc-whql\Packages\Drivers\Display\WB6A_INF\B176041)
to the miner folder, and all cards works well, even on drivers 15.5 beta