Against Win10 for ex., which OS doesn’t really matter. What I’m curious about is, if it’s worth porting a big game. So with AMD R9 Fury X / Doom you get additional ~ 40% FPS. How much of that would remain on OSX? Can you support all Vulkan features again like for ex. in Doom?
There are, of course, many factors that go into the performance of a game on any platform, so a blanket estimate of the variation between a game running on MoltenVK on macOS, vs the same game on Windows 10, using another Vulkan driver, is not something I can answer.
Your best bet is to determine the characteristics of your game, and build one or more test cases to explore those requirements in more detail, before porting the entire game.
MoltenVK is quite new, and depending on your requirements, you should also review the list of known limitations for the current release, which you can find in the README_MoltenVK_UserGuide.md document in the Molten download.