The Metal status saying 'Supported' vs GPUFamily1 v3 may be cosmetic. With my RX480 supportsFeatureSet:MTLFeatureSet_macOS_GPUFamily1_v3 returns TRUE no matter if the system report says 'Supported' or 'GPUFamily1 v3'. It also indicates it supports MTLFeatureSet_macOS_ReadWriteTextureTier2, which is a Metal2 specific feature set. The only time I've ever gotten the system report to say GPUFamily1 v3 is if my IGPU is also active. I've never managed it with the RX480 being the only graphics adapter listed in the system report. Maybe there is a way, but I gave up looking once I realized the Metal APIs were saying Metal2 was supported.