I think your analysis is completely spot on. Now, as you both said, there are probably some uncertainties in the data, and the exact training increase v. height slopes might not be completely accurate, but I think the pattern is clear. JS forwards seems to bite the big one if you're using it for tall guys. Of course, there is less of an out of position penalty for playing a big man in the PF spot. so I can see times when JS forwards is good to do. Plus, it's a two position training, so you can add more guys to the mix, which can be useful if you have enough young guys.
But in the end, for big man training, the best way to boost both JS and IS is a combination of JS guards/wingmen, and IS@C.
For short guys, you can probably get a decent amount of IS training through a combination of 1v1f and JSf. You'll still get more training on that individual trainee if you do IS@C, at the cost of half the training slots, and more out of position penalty to performance.