The "Overpowered" Leaders are to compensate for them being much weaker nations or as a reward for pulling off some feat. The Big 4 you mentioned are boosted by the fact they ruile over Majors and that is all the clout they need to be serious on the world stage.
Sultan Lady may have nice buffs but it doesnt take away the fact that Turkey is in the Firing line for 4 of the 7 Majors.
im really against op leaders too. you could argue they get a good trait for x thing but flatout 50% on everythign is just wrong.
op leaders should be the majors the legends of churchhill and rosevelt stalin are there because they were great leaders with aura.
hitler fixed the hyperinflation and could be reflected with 50% economic cost and stability
stalin ruled with an iron fist and could be reflected with compliance gain
churchhill was a great orator and increased the morale of the nation which could be stability and mobilization speed
hirohito was the divine emperor banzai was the warcry japanese soldiers never retreated they chose to die as last stands. last stance should be free and have 50%+ to organisation.
ataturk being op was reflected in history he was a great leader and the op status is balanced with him forced to die at 38
but giving u a even better nobody is just gamebreaking