I feel your pain on this. I bought this TV in July when my wife approved my 75 inch TV purchase (still boggles my mind how she allowed me to do this lol). I worked quickly before she changed her mind. :P I went and looked at them, I researched them and I spent $3300 on the thing I thought looked best at the time. Of course I could tell the SUHD TVs looked better but at this size I couldn't afford to step up any higher so I bit the bullet and have been enjoying it ever since.
Then that PS4 Pro event happened. lol Everyone started talking about HDR and suddenly I was starting to understand what it was. I figured I was all good since I had remembered reading a post here that we were supposed to get it via an update. Being an idiot I just figured it was no big deal and we could get it in software. After following this thread for several months it's quite obvious that this TV doesn't fully support HDR content. Yea, they released an update that will allow it to interpret HDR footage and skew some settings to give it a "similar" feel. But it's not the thing everyone is hyped about. I feel a little bit of buyers remorse, I'm not gonna lie. It will be a while before I'll be approved again for another expensive TV purchase. Haha. I could have easily had a 65 inch SUHD TV for less money but I really wanted a 75 inch screen.
So anyway... I've been thinking about it and wondering how fake or half baked HDR would look and feel on the 7100. I've been considering upgrading to the PS4 Pro as I play a lot of games there. I've not run any mods nor do I have a set top box that supports 4K or HDR. I've been watching 4K content on Amazon, Netflix and YouTube but none of it currently supports HDR. So I did a little not so scientific test. Haha.
Again... I'm not expert but I saw that Sony had a HDR 4K demo available for download (Thanks to someone who linked me to this site here. Sadly can't remember what the site was now). I had seen it previously on YouTube in 4K but it wasn't pulling an HDR signal of course. So I downloaded the movie file and put it on a data drive. Then I watched the HDR clip and the YouTube clip back to back. Yes, I realize they aren't going to be perfect because they are from different sources by stick with me. :P
I noted the settings when watching the HDR clip on the jump drive. Yes, the backlight jumps to 20 and the contrast maxes out. But the detailed color settings info I inputted into the TV remained. The YouTube clip was much less vibrant as expected. But it was using my old, standard settings. What if I just maxed out the contrast and backlight for the YouTube clip? Then everything could have this fake HDR look! lol I tried it and the colors looked washed out compared to the saturated and rich colors of the file with HDR on. It was clear to me that when the TV is pulling an HDR signal it's doing... something... to make the image look remarkably better. It was night and day.
So while this might not be "true" HDR the results were still pretty damn impressive. It's pulling in new color info that it didn't seem to have before. I also noticed that when playing that HDR clip a setting popped up in the picture menu for "UHD Color." It's not available when I watch 4K in Netflix (and anywhere else for that matter). It only appeared when watching the HDR signal. So while this isn't perfect it does give me some hope that HDR content will look pretty good on our expensive investments regardless of the fact that it's not full HDR 10.
I am still curious about a couple things tho...
1. Game Mode - Will HDR work with game mode enabled? I found a site with people who have our TV and the Xbox One S and they say it does as long as you set the Xbox to UHD Color.
2. When the TV is pulling an HDR signal the image does look really gorgeous. But how will this feel for an entire episode of a TV show or other content? Will the image feel too bright? Will I be burning my retinas out watching this thing like it's in torch mode?!
I need to see more HDR content to really know, I guess.
Anyway... just saw your struggle and wanted to let you know your'e not alone.
It's kind of rushed my decision on the TV and while I like it I don't think it's perfect. Hope this helps.