I tested again with yesterdays git and newer Kernel:
OpenGL renderer string: AMD Radeon HD 7900 Series (TAHITI / DRM 3.21.0 /
4.14.0-2-drm-next-dc-git, LLVM 6.0.0)
OpenGL core profile version string: 4.5 (Core Profile) Mesa 17.3.0-devel
(git-0c1aecf177)

The system doesnt hang anymore, it completes phoronix benchmark runs again.
Before it was always crashing when the 3rd game test was started. The benchmark
suite let the games run 3 times each test. So it usually crashed after 7-8
starts.
However I still see those GPU faults in the log.
Performance in Shadow of Mordor is still just 61-62fps.