If you can be a NBA firs round selection and elect to come back, this is why you should. Now I am not sure if Justin would have been a first rounder in the last draft but when he tested the water he was told do some things and you are sure to be a first rounder.
You come back because you have more talent that needs to be developed, you are not at your ceiling (or your roof). Justin was considered a good jump shooter, he worked on it, got more arch on his shot, reduced the hesitation in taking his shot and is now considered a great shooter and a sure fire 1st rounder. Up to this season Justin was not considered a go to type of player, more a complimentary type of guy, like a 3rd option but not the primary option. That is no longer the case, he has now shown that step up ability that he had not shown up to this season and that will be worth MILLIONS for him. It is what I hoped JMM and JP would have done because they as well had much more to their games than they had shown, felt they should have come back and improved those needed areas.
Justin has made his case now, I actually think he could come back and improve his case even more but not sure he could improve his draft positioning enough to make it worth the risk so I fully support and would advise the kid to grab that golden ring at the end of this season and wish him luck in doing so!