The brightness in this case is just the backlight. I would keep that maxed out for HDR viewing. For dim/dark room SDR viewing, this can be set much lower.
What is typically called brightness in most sets is is actually the black level in this set.
Black level and contrast (and backlight) may be set-up properly using a calibration set-up disc like the free one at AVS:
http://www.avsforum.com/forum/139-display-calibration/948496-avs-hd-709-blu-ray-mp4-calibration.html
Yes, the 5040ub is a fauxK projector that does HDR< but it has trouble from many sources, including HDR from the X1S from what I have read. It is by far the most affordable way to get a large screen at this quality though.
Another generation or 2 and I will upgrade my 1080p projector, but they have to iron out some of these kinks.
Interesting. So higher 'Brightness' in addition to Contrast being set to Max as per RTs settings? I noticed they didn't really touch on what happens in HDR mode.