Assuming the maps and assets are static, the only thing needing to be downloaded is the current state of the game and entities, which is a handful of megabytes. I agree that an SSD is completely irrelevant here, but so is the network. The hard problem here is how do you sync state between the different servers in real time so that transitioning between servers is seamless (and how do you handle reconciliation after an eventual net split).
From what I know, World of Warcraft does something like that on a conceptual level. Obviously syncing game state in non-action game like WoW is probably a little easier than something like a full blown action FPS.
Planetside 2 is a great and current example of multiple massive 100+ player battles occurring simultaneously in the same server/map.
Obviously its graphical detail is nowhere what was shown here, but the precedent exists, and the only thing that would be different is the geomotry and texture res.
