Why the heck would anyone want to upgrade the power supply in a 110V country? It's already pulling everything that's safe to get out of a 110V 20A outlet. I imagine it'll be a circuit-breaker flipper in some home offices, both because people plug it in to a 15A outlet by mistake (common in homes, uncommon in modern commercial buildings) and because people don't realize two outlets share a breaker and the other one has a coffee maker, a laser printer or another good-sized computer.
Another issue is things like vacuum cleaners and floor buffers that tend to be plugged in in random places for short periods of time. Leave a render running overnight and come in to discover that the custodian accidentally left a big vacuum in the middle of your scene! This thing is enough of a power hog that big configurations should be on a dedicated breaker (with other receptacles on the same breaker blocked off).
In terms of power supply quality, I can't see what could be an upgrade, short of some kind of lab power supply. This thing is going to be at least as good as the really high-end gaming power supplies (at the very least, it's something like a $500 Corsair, and I wouldn't be surprised if Apple's gone a step above that).
In 240V countries, there may be outlets that can supply substantially more power (without running something like a dryer cord). 240V 16A outlets are relatively common in offices in mainland Europe, for example.
Aside from T2 tricks, everything else should be upgradeable. The factory blade SSDs most likely aren't - nobody offers that upgrade on the iMac Pro, and that should be the same configuration. Using SSDs on a PCIe card is easy, and should even be officially supported (change secure boot settings to boot from them). RAM and GPUs are easy and officially supported. Unless they've changed something since the iMac Pro, CPU is possible - iMac Pros occasionally get CPU upgrades (unofficial).
The case where CPU upgradeability may be important is if Apple sticks to their guns on offering only the much more expensive high-memory versions of the 24 and 28 core CPUs. A small (and tech savvy) business may be able to save thousands by buying 8-core Mac Pros and 28 core CPUs without the high memory option. Simply put the original CPU back in if it ever needs to go to Apple for service. Big shops won't bother, it's not worth the hassle - but places with one or two video pros, architects, engineers, etc. who aren't afraid of the insides of computers?