Rajon Rondo, shots outside the paint:
Game 1 LOSS: 2/2: 100%
Game 2 WIN: 3/6, 50%
Game 3 LOSS: 3/8, 37.5%
Game 4 WIN: 4/8, 50%
Overall, Rondo is actually shooting a higher percentage outside of the paint than inside. The percentage doesn't really matter all that much though, because the number of outside shots he takes or his ability to make them don't seem to correlate to who wins. Rather, it's his ability to get inside and get easy baskets or draw defenders and get them opportunities for easy baskets.
This is where outside shooting matters - If he's making or taking those shots, that means he's not penetrating. The Cavs shouldn't care if Rondo actually is shooting well from mid-range and beyond. I don't know of a good way to track this other than anecdotally, but the real key is assists in the paint by Rondo. I imagine we would a see much greater correlation between that stat and the Celtics' success than what exists for Rondo's shooting percentage or even the number of shots he takes inside or outside of the paint.