This is ofcourse mostly dependant on the very internet connection you will have access too, once in the US.

But it is very possible to play WoW on EU servers from anywhere on the planet. There will be more 'lag' ofcourse the further you are away from the server. But even if you are on the other side of the planet, it's still about microseconds really. If you experience 150ms with a connection in the EU. You'd probably get 200ms with the exact same connection in the US.

I'm grappling figures out of thin air. But my point is, unless you got some really crappy internet connection in the US. There's nothing to worry about.