Just checked at the upgrade NPC in Valdrakken. There’s no explanation whatsoever of how the system works.
It looks like stuff that can be upgraded with Flightstones caps at 398. There doesn’t appear to be an option to upgrade any of it past that number. This means all of the flightstone stuff is garbage for me and I can’t use any of it.
That said, I haven’t actually tried upgrading flightstone gear to 8/8 and then checked if it can be ugpraded further. Can that be done? If so, it certainly isn’t explained. And that’s not really how any previous WoW currency ever worked.