You would think the amount of items rendered be the same so the performance shouldn't take a hit, but a 21:9 screen which gives you extra FOV alone is a FPS hit in it self, but the game might cull what you arent looking at.
But if you want extra FOV anyways and FF14 is a game you play a lot, I really do recommend a 21:9 screen anyways, check for online reviews, its an added extra cost but its one of those things that brings a smile to your face when you are using it.