Wouldn't it be safe to assume that if blade firmware updates/Thunderbolt/illumination/etc were part of an update and an Apple SSD was necessary to install them, then the update wouldn't allow installation unless the Apple SSD was installed?
I don't even remember the last time Illumination firmware was updated, Thunderbolt was at least two or three years ago, SSUAX blades had an update very early on. So, it's not common like BootROM.
Minor macOS updates install without you needing to go back to the original Apple blade.
The alternative seems less appealing. Updates can be installed with the NVMe drive, but blade firmware updates/Thunderbolt/illumination/etc won't be installed, and you will never know that they have not been installed. To be safe, you will have to install the Apple SSD before every update on the off chance that a blade firmware updates/Thunderbolt/illumination/etc is included in the update. This seems like a huge pain.
I don't think that it's sane or have a need to do that, just check MR, if we get a new BootROM, you will see it here. I always check the payloads, other people do the same.
The xar situation is temporary, Apple usually update it's internal tools before sending to everyone else, after that, business as usual.