Tony Allen was a starter on a good Memphis team, which I think is a safe description of a good team, and I think Smart is every bit as good if not better than Tony Allen right now, let alone where he will be at his peak.
I would be ecstatic if Marcus Smart turns into Tony Allen. In fact, if Smart becomes anything close to Allen, he'll probably be a 12-year starter on any team he goes to. To do this, he needs to learn how to make a bucket of some sort, because having a PG who can't make a jump shot or a layup is not a good start.
You are underrating Smart by saying to be more like Tony Allen, Smart needs to learn how to make a bucket of some sort. The Grizzlies have really struggled to score this year, due in most part to teams deciding not guarding Allen is their best defensive strategy. Smart is miles ahead of where Allen was offensively at the same point in his life (Allen was 23 his rookie year, Smart is currently 21). For Allen's first couple years in the league you would see him make mistakes in his defensive rotations, Smart hasn't done that since mid way through his rookie year.
Smart is already every bit of the defender Allen was in his prime. The difference is Smart is elite defensively at 21 where Allen didn't really become an elite playable defender until he hit his prime years.
Sure Smart hasn't put up good percentages this year, but I don't think the 9 games he has played are a representative sample size, especially when several of those game had Smart clearly playing hurt.
The biggest difference between TA and Marcus offensively, besides their style of course, is that Marcus takes shots and TA doesn't. But both bad offensive players. In term of style, TA was a slasher who could get to the bucket and finish. Marcus shoots a lot but he can shoot and he hasn't shown himself to be a consistent finisher either.
As for defense, TA was an elite defender from the get go. So not sure where that comes from. But to add to your point that the grizz struggle with TA on the floor, that is absolutely true. Problem is that will be true for the Celts with Marcus on the floor too. He may not be a guy who stays on the floor at the end of games because of it.
Allen was a very active defender from the get go but I think you are confusing activity with quality. He often was over aggressive and fell for up fakes early in his career. He is now widely considered an elite defender, but that wasn't the case before he got to Memphis. Had he been considered an elite defender at the time Boston probably is willing to pay the 3-3.5 MM needed to keep him.
As to your comment about Smart not being a guy who stays on the floor at the end of games, that is laughable. He has already shown the clutch gene.
Last year in the "clutch" time (last two minutes of games decided by 5 or less points I believe although I can't seem to find the NBA definition) Marcus Smart shot 53.8% from the field and 64.3% from 3. He is exactly the type of guy you want on the court at the end of games. For that matter his clutch shooting shows that he has the potential to develop into a good shooter imo.
As to Smart making a lot of defensive help mistakes, please pm me next time you see one so I can check it. I'll be waiting, but I don't expect to hear from you.