3DMark Time Spy
3DMark is the latest version of this hugely-popular synthetic benchmark. Making use of DX11 features such as tessellation, compute shaders and multi-threading, it provides modern-day results and is available as a free download.
Running the latest 3DMark Time Spy benchmark using DX12 halves the GeForce's performance lead. And educated inference would suggest that GeForce wins comfortably in DX11 but the Radeon comes back in DX12.
The same Strix cooler is used to good effect on both cards, and they easily pass the demanding stress test. The point here is that performance doesn't drop off by any reasonable degree once the underlying GPUs are subject to sustained gaming load.