Thank you for a thorough explanation, DonI'm somewhat new to the projector world (set up my first system a little over a year ago), but from all I've learned, you're right that the overall experience with the JVC should be dramatically better.
And yes, the custom gamma will give the best results with the RS400 for 4k/HDR.
Now, just to be precise, we aren't getting "true" 4k, due to eShift, so perhaps roughly '3k' vs '4k'. And the Wide Color Gamut of the RS400 isn't quite as wide as with the RS500/RS600 which have a physical filter to provide a wider gamut of colors. And due to inherent brightness limitations, televisions are able to provide a superior rendition of the wide dynamic range in brightness than virtually any consumer level projectors.
But within these inherent limitations, custom curves allow us to take advantage of all 3 dimensions of improvement available with 4k/WCG/HDR content.
Regarding the HDR to SDR conversion, 4k is maintained, as is WCG, but the HDR component is 'tone-mapped' to SDR. This is done either with the Oppo 203, which has a built-in feature to do this, or with other players, using the HDFury Integral. Basically, the player is 'tricked' into thinking the display device it is sending to isn't capable of HDR so that the HDR content is mapped into the narrower SDR range.
Generally speaking, the Panasonic UB900 with the HDFury Integral were generally felt to do this the best. The Oppo added this feature to its player, to accomplish this without needing the Integral, and initially did a poor job. They have improved it, apparently, but I'm not sure if they now do as good a job as the Panasonic. But both approaches will do a pretty good job.
As I said, before Arve's software became available, this approach of HDR to SDR conversion was the 'preferred' route to view 4k/WCG content on our Projectors. But now that we have this tool, this has mostly fallen by the wayside.
The only place where HDR to SDR conversion might still be desired is where the maximum brightness of one's system is significantly less than 100 nits. The basic idea here is that if one doesn't have enough available brightness, there just isn't enough 'room' to take advantage of the potential High Dynamic Range. This would be due to low gain screen, long throw distances, lower projector lumen output, etc.
But bottom line, even with the RS400, with Autocal and a custom curve, the 4k/WCG/HDR experience is pretty dang good! Add the overall image size which provides great immersion, and a good sound system, I now prefer to watch movies at home, finding the experience to be superior to any commercial theater I have been to, or have access to.
My instinct has at times been to default to just buying the proper gear and not getting too involved in technicalities - with the risk of missing out on the finishing line. This time I'm gonna bite the bullet and do the proper thing, not the least because of fantastic support from you and the knowledge in this forum.
I'm going with the JVC and will create the custom curve using the Speed Guide (although I don't have a Windows PC, sigh . I did a calculation with the Projectorcentral.com tool and it seems in theory I will get around 200 nits on the screen in my room = I will have some light to work with to carve out a good curve.
The Speed Guide mentions to use "the latest version of Arve's tool from the wip branch". Is that available in the Arve thread?