Personally I wouldn't listen to what someone from xbox support says as truth.



They don't always know what they are talking about.



Best to wait for official word from 343/Microsoft



Personally I've had hardly any lag and have even managed to win a couple of free for all games and i'm in the UK.



Now assuming Halo 5 uses the Microsoft Azure servers then they have servers in Ireland and the Netherlands which could be used for Halo 5.



My guess it that the match making system is designed to be fast so you get connected to the first server it sees that has people around your skill level. Whether that's in the United state or Europe depends on the circumstances. If this is how it works then it would be quite possible that an American could be playing on a European server and vice versa. This is a guess though and I could be totally wrong but after trying to play titanfall on an American server I know that I would always see lag if I was always playing Halo 5 on am American server.