So yeah, I just can't get past my wanting the U11+. As the owner of a Nexus 6P, I'm obviously able to get LTE speeds on T-Mobile, but how much would I be sacrificing in terms of speed and performance with the phone on HSPA+?
The specs indicate that the U11+ should be able to use LTE on T-Mobile. T-Mobile uses LTE bands 2, 4, and 12 (and possibly 66 and 71 in your area). The U11+ supports 4 and 12: https://www.gsmarena.com/htc_u11+-8908.php
HSPA+ speeds will depend on your area. Where I live, HSPA+ can give me upwards of 12-13 Mbps download speeds, which is pretty good. But in other areas, it might drop down to 1-2 Mbps.
In this review, what they say is that the phone won't be sold by carriers, meaning that you'd have to buy it directly from HTC outright, without being able to pay in monthly installments: https://www.engadget.com/2017/11/02/htc-u11-plus/. But there's no mention that the phone won't support US carrier bands.I've heard so called "experts" say it's not worth it because it won't support US bands.
GSMArena lists the bands the phone is compatible with. Assuming there are no weird regional variants, all models should have the same list. You can cross check with your carrier.
https://m.gsmarena.com/htc_u11+-8908.php
Won't it depend on if the carriers "allow" it to work on their bands?