Played around with the AMD CubeMapGen tool (check it out, it's cool!) and now I use a static cubemap for GI (very small and blurred), a bigger version with slightly more detail of it for environment map reflections (I use cubemaps from Emil Persson aka Humus). And, as I said, a HDR studio cubemap for the light reflections to make it glossy. Realtime, engine footage with FXAA:





Next: Screen Space Reflections for the plate... to mirror the rings!