Things to remember. If you have a projection system, even one with a decent tone mapping solution, you are basically just compressing that signal down to SDR levels.
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I think anyone that scoffs at the UHD versions that have projection systems are honestly doing themselves a bit of a disservice and just falling for marketing. Tone mapping and even wider gamut are rarely presented as intended on projection systems whereas you would actually have a true grade with the UHD version that is not some random interpretation of what someone else may think it should be with tone mapping and whatever the projector may or may not be able to achieve with wide color.
Just back from ISE, so bear with me if i´m on the wrong path.
Although i agree with Kris for users of standard projection setups, i think using the HDR-version with external tonemapping has some significant advantages.
You state the UHD version is pre-compressed to SDR levels. With "levels", i guess you mean nits? In this case, i´d say it´s mapped to 108nits.
So using the UHD version is indeed the best choice if your projection is hitting that mark or below.
But if your projection is significant brighter - with "significant, i mean 200+nits (regardless if you reach that with a high-lumen projector or just a smaller screen - or both) - you´d lose a significant part of the capabilities of your projection. You´d just throw away headroom.
With regards of the gamut, i wasn´t even aware of, that you are thrown back to bt709 with the UHD-version. Is that really the case? If yes, that would mean the UHD-version is in fact just the Bluray-version with 4K resolution.
I agree to the point that of course bt709 covers most of the colors normally seen in movies, but without a doubt, there is more to see. So again, if you habe a capable projection system, then you´d throw away another part of the capabilities of your setup.
I´m in no way argueing that everything else than a high-lument, high-gamut projection is rubbish - i just want to make the point that there are good reason to go for the HDR-version, also for projection.