That GPU ident is irrelevant, you can hex edit the kext to make it shows whatever you want.I can use HiDPI UI but it causes lags in scrolling through web pages and performance issues even in audio production softwares because CPU has to process more pixels
I don't know the exact reason but I think my GPU doesn't work well because it is not recognized well by the OS
also I have to mention that I use 3 monitors simultaneously one is 4k retina and 2 others are none retina 1080p
maybe this kind of problem is not so obvious in single monitor usage
For UI, may be Sierra still use CPU to process some UI animation / effect.
I never use RX580 in Sierra, and no plan to try that as well. So, don't know the difference.
But for Mojave, RX580 drive the UI really very well. Extremely smooth even it's rendering my Ui at 2x 4k HiDPI @144Hz.