I myself had issues with the demo/benchmark. Tried re installing driver, etc...
Found the issue for me was running two monitors.
R9 285, port 1 HDMI, secondary display LG LCD monitor @ 1280x1024
R9 285, port 2 DVI, Primary display Samsung 245BW LCD monitor @ 1920 x 1200
Primary display would white screen under Mantle when launching Star Swarm.
Disabled secondary display (LG monitor) and launched the demo under Mantle and the demo ran properly.
Don't know if the Dev's even look at this forum, but if they do please be aware of this issue.