Paul Pierce didn't let mere "good" athleticism stop him from greatness. He had a lot of the veteran guile and craftiness at an early age that served him throughout his career. RJ is similar in that regard. I think that's his ceiling.
RJ's primary deficiency at this point is his shooting. All of his other skills are good or even great. He can get more athletic, but gains will be incremental relative to how much he can improve his shooting.
In 11 post-all star break games, he averaged around 17ppg, 3apg, 5rpg, 45% FG%, 32%3pt%, 62%FT in 30mpg. He took 4.6ft attempts/game. If he averaged 70FT% instead of 62%, that would add another .5ppg to his scoring average. In general, FT% is a good indicator of consistency as a shooter and correlates well with 3pt% as well.
@NYKBocker - I think you underrate Pippen's peak athleticism. He was probably right there with Jordan among the tops in the league for speed/power/body control in his prime, all with a slightly taller/longer body. He had a remarkably long career and we saw his body breakdown over the years. The guy we saw in Houston and Portland was already 33+ years old. I guess you can say RJ Barrett has the athleticism of 32 year old Scottie Pippen - or a post-broken ankle Grant Hill.
When I think about benchmarks for SGs, I look to our very own Allan Houston and Latrell Sprewell as reference. Houston was an excellent methodical athlete. He was a former high jumper and debatably had a higher vertical than Sprewell. But he wasn't a fast twitch guy and couldn't make squirrel-like body adjustments mid-air. He couldn't gather himself for a quick second leap. He couldn't make his body react on defense like the elite guys. Sprewell was an elite fast twitch guy, extremely shifty. He more than made up for any lack of skills with a high motor and ferocious intensity. Sure he could jump, but I think we marveled at the quickness and power he brought to his forays near the basket.