It's actually not that bad at all. I've played from EU on NA centre a number of times and the game is playable just fine, of course occasionally there is packet loss which results in a weave not going off as intended but most of them go out just fine. It's definitely not a big deal for heavy ping jobs like MCH/NIN at least from my experience.

On a side note I heard land cables in NA are not the great in some areas, so often it can feel like connecting to EU from the east coast is more reliable to some degree.