there shouldn't be any limitations or performance hits from the radeon pro 580 not being on adobe's hardware list. adobe doesn't regularly update their system requirements web page, so there are lots of modern video cards not on that list. adobe has said any modern video card with 1gb of memory should work and that is usually the case.
I do have a new iMac with max specs (except 32GB Ram). But from my experience the GPU is not really doing anything. well compared to my old iMac it performs better, but GPU is running at around 5% and still with heavy effects (denoiser 3, which is GPU accelerated. and I tried effects from the supported list) it is not running smooth anymore, GPU stays about the same, even when switched off.
In AfterEffects I can not choose GPU for previews, even the unsupported GPU selection isn't available.
I can choose between open CL and Metal for GPU acceleration in Premiere but that doesn't really change much.
my first impression was, that it is running on the internal GPU from the CPU, but don't know if that's true and if, how to change that.
I called the Adobe Guys but they just said it's not on the List and I would need to ask here for help...
I do not know Mac's but some PC people have gone into the BIOS and turned the Intel GPU off to get GPU acceleration..