i havent read all this topic, but maybe it is not the salary and skill distribution, but sum of skill points that affects if training is still going good or slowing down.
SF needs some inside skills too, maybe thats the case why SF slows down there where SG keeps going on (if we look at salary). as we know the higher skill, the more salary you pay for that last pop-up.
in exaplme:
SG and SF, booth have 60 skill points, SG have JS-15 and JR-13, but SF have only JS-13 and JR-11. but as he is SF, those 4 skill points go to some inside skills. as we know, the bigger the skill, the more it costs for salary. so maybe thats the case why looks like SF stops earlier to improve than SG?
i havent trained SF, so i cannot compare. only chance is to search in market for similar players, but only older ones, because young ones probably have been trained and we cant see their real salary.