I don't think there is much more of a whistle for him to get. His FTr% is .499, higher than Luka (.498) and SGA (.476). Amongst real scoring options on their team, only Jimmy (.621), Deni (.570) and Paolo (.509) are higher.
But even if he did get up to Jimmy's FTr% (which is all-time), we're talking about 1.5 more FTA/gm, which at 72% FT% equates to 1.08 more points per game. That's significant on a team scale, but not going to be a huge needle mover on his TS%. His current TS% is .553, getting Jimmy's FTr would bump it up to .564. Not nothing, but also not the difference maker to making him an efficient scorer. Becoming more efficient will all come down to what we already know: improving his 3pt shot, continuing to hone that middy, and becoming more selective on driving into traffic.