I got 3DMark Time Spy DX12 free version (I own full DX11 Firestrike). Ran it on RX-480. It runs automatically at 1440p.

I got good, visibly-smooth frame rates with the demo and first graphics test. The second graphics test was smooth to almost smooth most of the time -- down to 10fps occasionally. The CPU test gave smooth frame rates.

The exact numbers mean little to others as we all have different systems. However, Time Spy benchmark is reported to have "far more" complexity than Firestrike -- and uses advanced GPU features. So, all in all, the DX12 visible performance of my RX-480 was mostly "very good to usable" in the benchmark. I assume the benchmark is tougher than a game -- I'll see eventually.