In v6 an AMD GPU should still work for rendering. In v7 (WIP) unfortunately OpenCL is no longer supported (due to lack of proper support from Apple).
Well, ViewCaptureTo* commands should still work in v6, and are much faster than the rhino-render-next plug-in even if they use the same render engine in the background.
The performance improvements we managed to do in Rhino WIP are the result of some pretty big under-the-hood changes that are not portable to v6.