It would definately lag like crazy if you had the max number of players, which is 756 x 4 sides = 3024.
However, would games with larger-than-usual numbers of players lag? I don't play MP too much, but the most I've ever seen in a game at once is like 30. Would a server be able to handle, say, 60 players at once? That would be PLENTY of players to have on at once. I doubt you could even ever get that many players together at once...
The big problem, of course, is the lack of join-in-progress, meaning you have to get all 60 players in the game lobby at once before starting. Using MP saving via Sinews of War, however, one might be able to swing something workable for large games like this. I would like to see some more expansive multiplayer as well.