I agree that 12 JS is not such a consistent level but, regardless of IS, with a potential 7 it's really hard to get more than that. JS 12, PA 12, OD 15 and the cap is almost gone, you can even try to find some cap from PA/OD but I definitely believe it's not the right choice. Maybe you can stop DR/HN to 14 to recover space for one pop in JS.
Let's assuming without additional training in IS they'll be like that
(https://i.imgur.com/hMkNqx5.png). If you're lucky, you have a high potential perennial all star and you might provide additional JS-JR...15-10 maybe. But, if this is a low perennial, could be capped there and his useless will be limited.
Providing IS, the situation will be similar
(https://i.imgur.com/VWUPVXd.png), if you're lucky the pops in JS will give you an excellent player. If not, this player will still have the opportunity to play a role in your team. Not in outside offences, but you cannot have everything with a perennial ^^