The 29 opposing coaches certainly don't seem to believe that Rondo's shot has improved. His shooting percentage is farce. I often quote that Ben Wallace had a very high shooting percentage but he is one of the worst shooters in the history of the game. Rondo is not as bad as Wallace but his statistical shooting percentage is no less misleading.
You will know when Rondo's shot has improved when opposing teams start covering him.
That's open to too many additional factors on the court. A couple years ago, if you were checking Rondo, you'd definitely give him some space--every team is going to take a Rondo jump shot over a Ray Allen jump shot, a Paul Pierce jumper, or a KG elbow shot. It's the least worst option, so you're going to sag off.
I don't have the tape on hand, but I'd suspect that they played a lot closer to him last season, while still honoring his passing, because a Rajon Rondo jump shot has less damaging effects than a Rajon Rondo assist.
Now, we saw this happen a few times; Doc crafting plays to get Rondo an open elbow jumper, which, again, is always going to be open for him, since the defenders don't want him getting the rest of the team engaged.