Competing isn’t really the target. V-Ray is a great engine, and if you want to use it you should. That said I don’t know if it is available for Rhino for Mac, or if it’ll be available for Rhino 6 on the Mac when we’re done. Regarding render quality I guess it is up to you do decide. https://www.cycles-renderer.org/ shows results with the engine. You can also browse the Blender community forum over at https://blenderartists.org to see work created with Cycles as the engine.
There is a great potential for materials, but it requires currently the geeky approach via the Cycles XML special material combined with GhShaderNodes in Grasshopper. With that your imagination is the limit.
Item 2:
OpenCL should work with AMD hardware. I have been unable to get the CUDA hardware working in our office, so that is still in limbo. Once I can get that running, I can fix and test Raytraced with Nvidia hardware. In theory it is great - in practice you are currently limited to CPU and AMD GPU.
Item 3:
See answer to item 1
edit: moved to #serengeti:mac since this is about the WIP version.