I see. 250 real players in the same map is very ambitious, even with UE3 games like Lineage2 implemented cross-server sieges with 100 players from each server, totalling 200 players in open world not instanced. While I think the player data came from different servers and they probably managed to have a separate server just for the sake of that specific event to occur, there were a lot of troubles, mainly latency from connections from different parts of the world playing. Events like instanced world boss raids, which takes more than 100 players, they still have issues with connectivity. Even the best solutions at someone disposal to implement a good game, would in the end reach the bottom line: people trying to play with underspec hardware and connectivity.
Would love to know in the future more things about your project! Cheers!