I sincerely hope Metal can solve the slow gaming speed of OSX. I will be the first to cheer if apple can equal or at least come close to directx performance using Metal.
I do prefer OSX to windows for nearly all tasks, but in games its just too slow. I have enjoyed video games since the early 80's, so I cant shake my gaming habit.. lol. When OSX games as well as windows, then I can finally be done with windows completely.
Macs have never been good gaming machines and will probably never be. As much as I loved my macs since '93 (switched from the Amiga which was a gamer's dream) I've always been frustrated with the poor choice of games and poor experience/performance compared to other platforms. I still had a good time playing countless hours on my macs, but it was never on par with what I could experience on my friends PCs (the PPC era was especially tough). I can get why developers don't care polishing a game for a platform with a 5% personal computer market share made by a company which doesn't target gamers.
The biggest leap for gaming on mac was the arrival of BootCamp (and the cMP too but that has come to and end), and it won't get any better than this. I can't see how metal would improve things. Who will develop games for metal appart from iOS devs (and god iOS games are mostly horrible grinding freemium empty nonsense) ? Most titles will still come for console/Windows and DX11/12 and some of them will be ported to Apple APIs with more or less care as it has always been. These ports often come with the usual performance degradation, missing graphics features, glitches and unpracticality (impossible to switch from application, unaware of HiDPi displays, overriding function keys, preferences and save files in odd locations, limited controller support, etc). Even Windows which comes with decades of gaming performance refinements has the same problem with console ports, consoles representing the largest part of today's gaming market and the main target for game studios.
Some companies do treat the mac gamers audience with love and end up with a product that performs mostly the same on both platforms and has the "mac feel", like games from Blizzard, ID Software or most of the SimCity and Civilization series for example, all made by studios with a more than two decades experience and commitment developing games for mac from the ground up (as opposed to being ported afterwards by Aspyr or Feral, which still makes a decent job out of it). Some performs also better on macs because they were developed on mac like Prison Architect (the only mac game I played that's aware of HiDPi display), but these are and will be exceptions until the mac represents a decent part of the gamers market.
If you're serious about gaming (serious enough to buy a gamer graphics card like the 680), there is one instant solution to most of your performance and graphics quality problems, get yourself a Windows partition. That's the single biggest and cheapest performance upgrade you can offer yourself today and with SSDs rebooting is not the pain it used to be. Or you can wait and hope Apple and game developers start really caring about mac gamers like I did for years. Just don't expect a system update or the metal API to fix everything like a magic wand, this has never happened despite numerous hopes in the past. It will more likely break compatibility with some of your older games, and with a yearly major system update cycle this happens pretty often.
Performance tuning requires a lot of dedication and knowledge from devs, Apple and graphics drivers providers for each game on each piece of hardware and there's no other way around it. Metal will give devs more opportunities to fine tune their software given they are willing to spend the effort first. Unfortunately few people are ready to invest this time and money for the mac platform today, especially given that all macs can now run Windows.
