I can't speak for cycles, but Redshift is very slow on OpenCL. I've never heard of using it on Metal. I know it was written for CUDA and didn't even support OpenCL for a long time. Do you know how it runs on Metal?
I don't think V-Ray GPU will run on anything but Nvidia, and if it will then it's definitely slow.
I don't think V-Ray GPU will run on anything but Nvidia, and if it will then it's definitely slow.