Running AMD driver 14.4 gets me 580 h/s.

Copying the AMD opencl files from the 13.12 driver package to the same folder as the GPU miner gets me 810h/s and less error messages, while still on 14.4 driver

Keep your new driver, download the old 13.12 driver from
http://support.amd.com/en-us/download/desktop/previous?os=Windows%207%20-%2064
and only run the first step of the installer, that unzips the files to the c:\amd folder, then exit the installation.
Then copy the amdocl* and amd_opencl* to the GPU miner folder from
c:\AMD\Support\13-12_win7_win8_64_dd_ccc_whql\Packages\Drivers\Display\WB6A_INF\B165829\

Would be nice to have the files included with the miner, but I guess there is a small copyright issue...