Barefeats has benchmarked several Macs and hacks on this game.
http://barefeats.com/imac5K_metal.html
I'm disappointed by the difference between macOS and windows 7 (hence DX11) on the same machine. The difference can be as high as 60%. However, this comparison was made on nVidia cards only, which have beta drivers (1080Ti) or at least drivers that are not known for their good Metal performance.
I've compared hitman on my iMac and the Metal-DX11 difference is modest. I also tried War Thunder. The difference is quite large (windows is at least 50% faster), but the game appears to be limited by the GPU even at low resolution on macOS. The CPU is used at less than 50% (I made sure the framerate was below the 120 fps limit of Metal). Metal is supposed to reduce the work on the CPU, and appears to be working as is should. I'm not sure why the GPU generates fewer fps for the same load on macOS. Less efficient drivers, poorly translated shaders during the port to Metal...?
There are games where the macOS-windows performance reduces when it is limited by the GPU hardware (typically at high resolution), which makes sense to me as it doesn't really depend on the OS at this stage.
But there are games where the performance difference increases at high resolution, which I cannot explain.