From the interviews with Yoshi-P it's pretty clear that a third party is responsible for the Mac Client, and logically speaking, every time there is a patch for the Windows version, the Mac version has to stay in lock-step, so it's reasonable to conclude that the OS X client is simply the Windows client with a graphics shim somewhere in the rendering layer, and that's probably the reason for the poor performance. Anyone who has ever tried to run a Windows game on Linux or Mac OS using Crossover knows what a pain in the butt this is.
This answer you pulled completely out of your butt.
MacOS X is far easier to develop for, but you're overlooking the fact that Square Enix also has a PS4 version. All versions write to some common graphics layer, and the Windows version writes this to DX9 or DX11, the PS4 version writes this to OpenGL or GNM (I'm not a SE Developer,) and so does the OS X version use OpenGL. There is no direct 1:1 mapping of DX calls to OpenGL , because it's comparing a C++ COM API to a state machine API. It's like comparing a rock to a watermelon, they're both round, but that's about where the similarities end.
Japanese developers are just as familiar with MacOS X. The key thing to consider is that a lot of Japanese software (eg doujinshi stuff, and MMORPG's) are written for Windows because that's the "Adults" computer. The game console is the kids computer. Both the Famicom and the SG-1000 were marketed as computers in Japan while they were marketed as toys in the US and still largely are. So the reason you don't see a lot of Mac OS X games in Japan is the exact same reason it is in the US, everyone builds their "Kids-to-Teens" games for the console and leaves the Windows PC for the Teen to 18+ market.
The market for games is on the PC. It's a catch-22 situation, you can't get people to game on OS X if there's no games, and you can't get people to switch to OS X if their games aren't on it.
Indie developers are far more aware of the need for OS X and Linux versions of their games, but they don't build non-Steam versions of these games because Linux users don't buy games and have awful GPU drivers, and Mac OS X users tend to not have dedicated GPU's. So nobody wants to port a high-end title to either platform because there just IS NOT any high end hardware support to justify it.
Kudos for SE at least trying. It might have made better sense to use have it steam-only license and utilize the PC/Mac version separation on Steam than release a separate license entirely for Mac OS X.