I would consider quite=completely to be archaic, which would fit in with American (and to a lesser extent, Hiberno-) English using it that way.

Actually, I use it more as an intensifier when I'm agreeing with something ("Oh, quite." or "Quite correct.") than when I'm making a statement of my own.

Ah, the pitfalls of an unwary right-pondian attempting to write fanfiction in an American canon! Although I think we have the advantage over the Americans trying to do the opposite.