After further review and a solid week of seat time with my new XBox Series X, I've reached the following conclusions:
1.) I now have two older XBox One games which have been elevated from idle (NASCAR HEAT 4), to not owned based on a negative play experience on my One X (Assetto Corsa Competizione) to "regular play rotation".
NASCAR HEAT 4 was kind of like PGA TOUR 2K21 in that it was capped at 30 FPS, but all too frequently dipped even below that. On the Series X, it's rock solid 30 FPS and free of any hint of judder at any time, even on the road courses. It also looks a lot better with the Auto HDR at work.
Assetto Corsa Competizione went from unplayable on the One X, due to frame rate struggles, to amazingly smooth on the Series X. According to Digital Foundry it went from 34 to 52 FPS, and it feels every bit of that. This also helped the steering out considerably, even though it already had one of the best force feedback implementations I've ever experienced in a console racer. The Auto HDR worked a miracle on its vanilla looks to make it look more like a next-gen racer, although it's still not in the same camp as Forza 7 or Forza Horizon 4.
2.) Project CARS 2 went from ho-hum, play once every other week to a game that feels almost like Series X optimized! Again the frame rates are solid and I can run it best visuals mode and it purrs along. Like ACC, it's force feedback implementation somehow seems to get an intangible but welcome upgrade
3.) Rainbow Six Siege: the frame rate definitely improved, but I still suck at shooters at 90 frames per second as I did at 50! I couldn't really detect much of a difference in the graphics between my One X and Series X, but this isn't a game I play for fun, I play it more to produce bench test results. It will still not be on my play list, post Series X.
4.) Forza 7 didn't seem to get a boost of note, but they haven't really tried based on all I've read. It was already outstanding in 4K at 60 FPS on the One X. I can't wait to see what they do with Forza 8, optimizing it for the Series X!
5.) Rocket League: I couldn't tell much of a difference on the Series X to be honest. I went from performance to quality graphics mode, but it didn't result in a chance that I could perceive. Still a fun game and plays smoothly enough, but I honestly couldn't tell you what platform I'm playing it on.
6.) The Golf Club (OG)... that's right, in my geeky world I had to go back an see what the original XBox One version of The Golf Club looked and played like. It got no discernable help in any department that I could detect, outside of 5X faster load times. 900p video upscaled to 4K still looks like 900p... I'm sorry. Just for kicks I tried the "vsync disable" option in the settings and it made no difference in the languishing frame rate. I was told that they actually removed the functionality of that option, even though it still shows up as an option in the menu. IOW, you can turn it "OFF" but it's still really ON.
7.) Elite: Dangerous got a bump in looks and performance. I can run with hi-res and quality and get smooth control. It also looks better in all departments. I'd still love to see what they can do with this one optimizing it for the Series X.
8.) Finally but certainly not last, PGA TOUR 2K21: I was hoping that the Series X was going to prop it up to the point that I could go back to having it be my base platform for the game, but it ain't happening. Yes, it looks better with the auto HDR, and gets along faster with the quicker load times. It also seems like the stick is more responsive to inputs, but a more responsive stick at 30 FPS is still going to feel like swinging a club underwater after you've acclimated your senses to doing the same thing on the PC at 60 FPS.
I just can't get past my disenchantment with that, especially knowing that the the console on which I'm relegated to 30 FPS is 20 to 25% more powerful than the PC on which I'm playing it at 60 FPS. If I'd never been exposed to the PC version at 60 FPS, I'm sure I'd be at least content (but not enamored) with the game on the Series X.