Thanks for the info, James. My main (mixing for DP and video editing) Mac has an AMD Radeon as well.
It's not fair to compare an NLE to a DAW in video respects, as NLEs are using the GPU more and more as developers evolve the products. Premiere has an option to use the CPU or GPU for rendering effects in real-time as the video plays. Some effects are written for GPU and some for both.
I was a little surprised to see somebody raving about Metal, due to my circumstance. I don't play video games, which generally benefit from advances in GPUs and their rendering architecture. When DP is playing, my GPU meter hardly registers. I surmise it's doing the GUI animation, and that for plug-in GUIs that display a spectral graph, etc.
It's possible that the Premiere coders just haven't figured out how to get max performance from Metal vs. OpenCL, and the weak link isn't Metal.