You can host 250 players servers on NW...
It is very well possible to do so, and should not lagg. We never tried, because Taleworlds/Paradox did not want us too, but the original plan was to actually support up to 300 players.
But yes, you can host 250 player servers on NW the same way you can do it in Warband. With a simple hack of the server binary.
And btw... That Native Siege server was ours. We were the first one to actually host Servers with more than 64 players. So yah. I am pretty sure, even though I never tested it, that NW should very well work with 250 player servers.