Radeon ProRender shows R9 200 series (R9 280X) available for rendering but it does not work.
Here is installation of the Radeon ProRender plugin for Blender 2.79 result:
And here is a picture showing the Prorender Menus activated in Blender 2.79b:
Before anyone get's too excited because they see the checkerbox for an R9 200 series GPU - it's an 280X (aka HD7970) (GCN 1.0) GPU shown ticked... The GPU does show activity in Wattman (i.e. it seems to be working) if I use it in Radeon-ProRender along with the Fiji GPU's to render.
However here is the render result if I just run Radeon ProRender on the R9 280x.
A veritable camoflaged polar bear in the snow. It's broken or unsupported on R9 280x GPU.