The Celtics were 8th in the NBA in point differential last year, they kept all their major pieces from last year, added a top-flight center. OKC, and Atlanta got appreciably worse, the Clippers and Spurs got a little worse, Cleveland and Toronto mostly held serve, the Warriors obviously got much better. If they add Westbrook, I think it is clear that they would vault past everybody but the Cavs, Spurs, and Warriors. An argument could be made for the Cavs, Spurs, and Celtics to be anywhere between 2nd and 4th best teams in the NBA, but with the rest that will be given to Lebron and the Spurs players, I think the Celtics would have the 2nd best record in the NBA.
I think I just talked myself into making a trade for Westbrook if it only costs the BK pick and Thomas.