There is no reason why rx580 would not be supported on ARM or why there would be any meaningful performance delta. AMD does not have any kind of “secret-sauce”

There is no reason why rx580 would not be supported on ARM or why there would be any meaningful performance delta. AMD does not have any kind of “secret-sauce” driver for that, it is simply LLVM targeted to that architecture that converts HLSL/GLSL/SPIR-V into the architecture specific code.