Saying that the bandwidth increases by an exponential factor for every player added I would think is only true if your application is extremely unoptimized.
Optimized for what? Which is faster, just sending all the data, or figuring out which objects can see other objects? Traversing a dense graph, is very time consuming (at best polynomial time, most likely factorial time). why would you update every player in a 200x200 map when the screen is only 30x20?
Because doing the math to figure out, for each player, which other player is within their "area" is an NP-complete problem, not solvable in polynomial time? Sure it works well for small amounts of players, but 3000! is a HUGE number. Even if checking every object in the game world only took 1 CPU cycle, calculating the exact solution, would take forever. Best way to send packets is using SSPP
A what? Basicly that meens ending a short with the lengt of the packet. The packet is send in bytes. Done.
The UDP header already contains the size of the packet. Adding redundant data doesn't seem like it's an efficient thing to do.