Looking at all star wings and SGs the last 5 years at similar ages to RJ:
Paul George: 90%(!) FT shooter his last year at Fresno State, though to be fair, "only" 70% his freshmen year. Averaged ~40% from deep over his college career.
Kawhi Leonard: The prototypical example of improving your shooting, still a 72% FT shooter his freshmen year, and 76% when he graduated
Kevin Durant: Good FT shooter at age 18 as expected, at near 82%
Klay Thompson: 90% FT shooter freshmen year
Victor Oladipo: legit bad shooter at age 18, 61% FT and 30% from deep. Maybe RJ's only lifeline here. Though he got it up to 75% the very next year, so maybe it was more of a fluke.
Bradley Beal: 77% at Florida.
Khris Middleton: 75% at age 18
D'Angelo Russell: 75% at OSU
Dwyane Wade: "Only" 69% at age 18 at Marquette, got it up to 75% the next year
DeMar DeRozan: Another mediocre FT shooter at 65% during his USC year. Probably not the guy you want to be as a "franchise talent"
Jimmy Butler: 77% from the line age 18
Gordon Hayward: 81% age 18 at Butler
Carmelo Anthony: 71% at Syracuse age 18
Kyle Korver: 90% guy age 18 at Creighton
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Seems like the biggest leap occurs between the age 18/19 to 19/20 season. We'll know real, real fast if RJ is ever going to make anything of himself in terms of becoming a shooter.