In the past 20+ years, what Iowa player have you witnessed that has improved his perimeter skills more than Marble did? I happen to think that making such a prediction in a player's development is unrealistic.
Further, there is a large difference between improving to be a good player, and improving to be a star player that become First Team All Big 10 and NBA draft pick. I never questioned why Marble was brought to Iowa. I always expected him to become at least an average Big 10 player.
Overall perimeter improvement? Probably Jok, his Junior year certainly he appears to be passing by Marble's level. The last twenty-years by the way doesn't give us many good players to pick from.