Well it's not really a big deal since only your UX gets a smaller range... when you're presumably watching the content?
But it seems like Apple might be gradually moving from 8-bit to 10-bit displays. They don't really advertise it clearly, but if the technical specifications say "millions of colors" it means 8-bit, if they say "billions of colors" it means 10-bit. (256^3 vs 1024^3.)
So if you've got the iMac model with the 4K screen, it's 10-bit and therefore capable of 1,024 levels of color. So remapping the UX to e.g. 512 will still be fine, assuming (safely, I think?) the compositing is all done using 10-bit color.
But it seems like Apple might be gradually moving from 8-bit to 10-bit displays. They don't really advertise it clearly, but if the technical specifications say "millions of colors" it means 8-bit, if they say "billions of colors" it means 10-bit. (256^3 vs 1024^3.)
So if you've got the iMac model with the 4K screen, it's 10-bit and therefore capable of 1,024 levels of color. So remapping the UX to e.g. 512 will still be fine, assuming (safely, I think?) the compositing is all done using 10-bit color.