Kind reminder: Metal exists now, and that's what Mac games are generally using from now on instead of OpenGL. While that doesn't necessarily mean that performance is equal (and indeed Blizzard games with Metal renderers still tend to lag behind Windows performance), so far the fps hit is usually much less.
"Windows side" is not something that just automatically exists. Since I believe everybody is aware of the possibility of buying Windows and using it with Bootcamp by now, and indeed has been for many years, it seems pointless to mention this. We can just assume, for any game, that anyone who is willing to use Bootcamp has already done so. The discussion is about Overwatch on macOS.
--Eric