Properly set up, 4K and HDR at 45 foot lamberts looks great. Not everything looks to the eye that much brighter than at 34 foot lamberts. It's mostly highlights and certain parts of the picture ) depending on the scene and movie ). The Matrix looked excellent at 45+ foot lamberts. And my wife has not once complained that it's too bright !To me it seems that actually HDR kills the black levels.
Projectors have a fixed black floor that is restricted by the lower Native On/Off and the peak white.
I'm not sure how this looks at 40 fL or whatever, but I can guess and it's probably not great.
HDR cannot solve this problem, as HDR increases the range of everything just above the floor up to the peak white being brighter.
That doesn't change the actual contrast ratio for bottoming things out, that just makes the intrascene contrast higher in certain scenes from the adaptive gamma.
It is only because of silly high contrast ratios with oLED that some TV's can somewhat overcome the issue.