To check if it is a hardware problem or a problematic bios setting I installed
openSUSE 13.1 in parallel, there I was able to test with fglrx. And it is
working well, the power management is working and the clock is getting up to
the max when running glxgears (tested with aticonfig --odgc).

So this has to be a driver bug... Can you give me any hint how I can debug this
problem? I'm a software engineer myself (but mostly Java) and can test patches
or "play" with the code if you say me where... ;)