So, I've gone ahead and purchased the AT&T Note10+5G and thought I'd share my experiences for others who might search and find this thread:
First of all, AT&T is clueless. Once I had the phone in hand, I noticed that I wouldn't get a real 5G signal. I called AT&T about 6 times for this issue, and each rep would tell me something different. To make a very long story short, AT&T had to specifically provision the line for 5G, which resulted in their system pushing a new APN to the phone. Instead of "nxtgenphone", the 5G phones need a "enhancedphone" APN.
Once that was done, I was able to get a "5G" (not 5Ge) signal. However, speed tests under "5G" are no faster than speed tests when the phone reports "4G LTE" or "5Ge." This, actually, isn't unexpected. What's happening is that AT&T (to the best of my understanding right now) has only allocated a 5Mhz part of the 850Mhz band for 5G. The phone will only select that 5G signal when the 4G/LTE/5Ge signal would be worse (which, only has happened for me when I'm in a very congested area, such as a shopping mall.) In those places (and at those times) the 4G speeds would be horrible. For example, less than 30mbps. At the same time, the 5G speeds would be closer to 100mbps (only because no one else seems to be using the band.) Once more people are using AT&T's lowband 5G, I'm sure they'll allocate more bandwidth for it.
In other words, right this minute, AT&T's low band 5G is little more than a trial run and/or marketing gimmick.
All that being said, I'm still glad I got the phone. The reason is that I seem to be getting much better 4G reception with the 5G phone than I did with the non-5G Note10+. I'm not referring to the silly "bars" at the top of the phone, but to the actual signal strengths reported by apps such as CellMapper (and the fact that the 5G phone is able to hold a connection in areas where the non-5G phone would go to "no signal.") My only guess is that the 5G phone is using the X55 modem for 4G as well as 5G connections, and it's just a better modem than what is built into the snapdragon 855 chip. Or, perhaps, there's a different antenna placement on the 5G phone. Whatever the reason, it makes the phone more reliable.
Sadly, it appears that no one has done a tear down (or even x-ray) of the X55 variant of the note 10+5G (SM-N976U.) The closest I can find is a teardown of the SM-N976V which uses a completely different modem and antennas.)
As far as battery life, I can't tell yet. I'm still in that "first week" period of having the phone (and all android phones tend to have miserable battery life for the first week after purchase or after a factory reset.) I'm going to assume that because the hardware of this phone is identical to the hardware of the TMO variant, that the battery life will also be similar.