This thread has been incredibly illuminating, and reinforcing that I'm by no means alone in this.
Many thanks to the previous posters on this thread! Upon AosiClericsguard's implicit advice, I asked for a refund. While I didn't get an e-mail as cool as AosiClericsguard's and KrelianJudah's, I did receive this:
Square Enix Support (Square Enix Online Store Support)
Jun 25, 16:05
Dear [name],
Thank you for contacting the Square Enix Online Store Support.
Your order has been cancelled, and you'll be refunded within the next 1-7 working days.
Best,
Xavier
The Square Enix Online Store Team
(In case people are curious: My machine is a 27" Late 2013 iMac with the proper OS and more than enough hard drive space, and an NVIDIA GeForce GTX 780M graphics card.* My fps in Limsa Lominsa during seriously off-peak hours for my server ranged from 25 to just under 10.)
This major plus side was after a pretty serious minus side of having been online with Square Enix's Customer Support Chat for . . . nearly four hours? During which I spoke to a Customer Service agent (who didn't fully elaborate until 1.5 hours in that he was Operations and Billing, could not offer me any tech help nor refund me for the game -- and in fact claimed it was not possible to refund the game), and a Technical Support agent who -- I kid you not -- suggested that contacting Technical Support was not the way to receive technical help with the client and that I should instead "please post on the forums when you are able" because "Mac is a new platform for us and we are actively investigating all reported issues."
So seemingly, this is the place to report Mac issues. Report away, all!
(They also told me to follow up by posting on the forums when I was also contacting them because I initially couldn't post on the forums due to a forums permissions bug. And responded to non-responsivity in the Report a Bug window in the Mac client -- I found this out because the Customer Service rep told me to address the forums permission by reaching out to a GM in-game -- by telling me to try to "please attempt to try reporting a bug on a PC or on the forums once [your forums issue has been resolved]".
* - Ironically, I bought the Mac version because several folks have found the NVIDIA GeForce GTX 780 drivers to not always play nice with Windows 7 Professional under Boot Camp; like many, I experience serious occlusion, tearing, and redraw issues even when system windows pass over one another. It's a known issue, and most people 'fix' it by turning off NVIDIA updates from the native NVIDIA updater as well as Windows and Boot Camp updater, and rolling back driver versions until they find the one that sucks the least. But even these graphics issues don't compare to the fps I was getting.