On maximum settings – 2560×1440 Ultra with 8x MSAA – both machines average about 20 FPS in the built-in graphics benchmark.  And the AI benchmark yields similarly indistinguishable results, with average turn times of about 23 seconds.

With VSync off there’s maybe a very small increase in frame rate – closer to 24 FPS, than 20 FPS, but it’s visually indistinguishable to me.

And that’s despite the iMac Pro having, in addition to the beefier CPU & GPU, use of its very fast built-in flash storage, vs a regular SATA SSD on the 2014 Retina iMac.  Load times were noticeably faster, but only in the 10-25% range perhaps – nice, but not impressive all things considered.

In actual gameplay, I did feel like the iMac Pro offered a smoother experience overall – maybe not higher average frame rates, but fewer stutters or skips.  Hard to say, though, if that’s a real effect or just a misimpression.

So, definitely don’t buy an iMac Pro for playing Civilization VI – any iMac from the past four years will be just as good, sadly.  Whatever Civilization VI is bottlenecked on, it’s clearly not the CPU throughtput, GPU anything, storage anything, or memory anything.  Which I guess just leaves CPU single-threaded performance. 😞