I certainly bought it for gaming and couldn't be happier. Of course I also bought it for using OS X, iCloud, and Logic Pro but gaming was most definitely still a bullet point for me. Which is why I'm hotly anticipating the new iMac this year and I'm very interested to see what GPU Apple ultimately decides to use.
And I'm not sure what you mean when you say you buy the iMac for the "best possible OS X experience." I think a Mac Pro would probably run OS X the smoothest, and running it in a portable environment would obviously be best served by one of the MacBook products, which in fact, based on sales, is what most Mac users actually prefer. Each individual's usage of OS X will vary so it really depends.
Generally speaking the iMac handles some games well, but let's face it. The Mac is not a serious gaming machine. Most of the games on the app store are ok, but they do not constitute even a small selection of the most outstanding action, adventure, simulation, strategy, sports, racing and so on, games available for Windows. If you want gaming, you buy Windows. If you're a very casual gamer, the Mac is fine. Though you can boot it under Windows and have access to most games that way, you'll always be behind in the latest performance hardware available. Best to build yourself a nice machine where you can even swap out the CPU and the GPU when you want to.
This is the reason people such as the CEO of Oculus (the virtual reality company) asserts that he will support the Mac when Apple builds a good computer. ( Or something to that effect ). And before you dismiss him as a jerk, consider that long time Mac gaming proponent Blizzard, the makers of World of Warcraft, recently dumped the Mac as a platform when introducing their new game Overwatch, which is the current big craze. Blizzard basically said, in no uncertain terms, that it's too much trouble, the hardware just isn't there, the software just isn't there and Apple provides no help. Now I've seen Overwatch running on an iMac booted under Windows, and it's just not as smooth and the graphics are just not as breathtaking. It's a relatively mediocre experience.
Concerning best possible OS X experience... The Mac Pro is an odd duck to say the least. It's built using "Workstation Class" hardware, Xeon-based CPUs, and Dual FirePro GPUs that make the Mac Pro perform best in a highly select situations and environments. Applications have to be designed to take advantage of the Mac Pro's hardware and most are not. The iMac outperforms the Mac Pro in gaming because the Mac Pro's GPUs are geared toward workstation tasks, while the Radeon's in the iMac do better in the world of games.
According to Computer World "...The Mac Pro and the iMac can both run Final Cut Pro and Photoshop, and you'd expect better performance from the pricier Mac Pro. But the iMac's hardware supports OpenCL -- as all shipping Macs now do -- and, the iMac even beats out some of the low-end Mac Pros in specific benchmarks..."
Now add to that the iMac's 5K screen, which is the best screen on the market, especially for people who stare at them all day, with that 5120x2880, 14.7 million pixels crammed into a 27" monitor and it's pure visual joy.
The iMac is an all-in-one computer also. No need to go out and buy a monitor.
So yeah, generally speaking the iMac is the best possible OS X experience... for most people. You really gotta have a specific need for the Mac Pro.
You are correct when you point out that I excluded portables. I did that because the original question was about the current highest end GPU coming to Macs which just ain't gonna happen.