Afaik, playmaker SA has same - equall effect which is independent, I mean, not determined by players position. It will give some small ball posession bonus, so has equall effect on both DC-DMC or MC/ML/MR.
For example, Free kick SA or corner kick SA won't have equall effect on striker, mildfielder or defender. Very few defenders with FK SA would actually score goal from free kick. Also I've noticed, don't know why, corner kick isn't that effective on MC as much as on wide or attacking mildfielders and strikers.
What I've recently found out is that my ML-MC dribbler decided few matches where he managed to dribble alone few players and score (saw comments of his action during the match). I though this SA was completly uselles and ineffective, but I was wrong.
And again, afaik, shadow striker should increase chances of scoring goal from distance shoots. So, if this is true, then that SA would be more useful on MC than on AMx. I had in Dnipro last season 5* AML-AMC-AMR shadow striker, but he scored only few goals although had good form. So if someone has AMC-MC or wants to train such player, I'd suggest to train shadow striker if that player is going to play MC as his default position, and sometimes AMC. This way shadow striker should have more effect, because ML-MC-MR usually shoot form distance, more often than AMC.
Of course, if someone thinks i'm wrong, then i'm glad to read reply and learn why this isn't correct.