Did a quick comparison today on Dota2. The moltenVK version is not faster than the openGL one at 1080p on my 5K iMac running Mojave.
At >4k (4096*something), it's ~25% faster than openGL
Vulkan/DX11 on Windows is ~55% faster than moltenVK at 1080p. Clearly, the translation layer comes at a CPU cost.
The openGL windows version appears bugged. Too bad, it would have been nice to compare to macOS openGL.