The RX Vega 64 / 56 cards dominate the 99th percentile frametime tests for all resolutions in Forza 7, while the GTX 1080 Ti only manages to stay on top in the 4K FPS test.
https://www.notebookcheck.net/AMD-s-RX-Vega-64-56-GPUs-are-considerably-faster-than-Nvidia-s-GTX-1080-Ti-in-Forza-7-DX12.255736.0.html