Just want to correct you one thing.
If you read my previous post carefully.
It is NOT necessary to have a ID15+ center to stop a 15 JS bigs. 9 ID at PF or 7 ID at C is enough because of the penalty.
Since you are still training him, I would suggest you add 2-3 IS then he will be truly lethal.
You mean OD, not ID, but just pointing that out would be a pointless post. ;)
I think you're confusing the "penalty" to team ratings (like Offensive Flow, etc.) as calculated by tools like Coachparrot with an actual individual player penalty reducing the players' skills because of the offense/position they play in. The fact is that the team ratings in Buzzerbeater are an overview but not the end-all and be-all (unlike Hattrick, if you play that, where the ratings themselves are used in the engine calculations).l
The easiest example I can think of is having big men with good passing/handling/driving skills, and then replacing them with guys with all 1s in those skills. The offensive flow doesn't change, but you sure can see the results in game - far more turnovers and far fewer assists between big men.
My experience is completely the opposite of what you're claiming in terms of JS at big. My trainees have nowhere near 15 JS yet - one's a 10/8, one's a 9/8 and one's a 7/5. The 10/8 guy for a while was among the league leaders in 3pt%. While it doesn't take equal OD to hold shooting percentages down at any position, a 7OD guy is going to be torched by a 15JS guy.