No. Not at all. In no way.
Is this sarcasm?
As Larsenex said, if the AI can use them (and it does) then it's not overpowered.
To be honest, this logic is not particulary useful.
If the only thing required for balance is, that eveyone can use it, you can get situations where the game provides ten ways to play it, one of them is superior to the others. So the only way to win is using one out of the ten ways. Players are balanced against each other alright... but game styles are not balanced against each other and there is a lot of unused potential.
To get back to the question about leaders:
Lets say I have 5 planets (which is not a lot). So, a leader will give me 5 research (and some %...). A researcher will give me 24% more research bonus than a leader to one planet. How much flat reasearch does the planet have to have in order for a reasercher to be better than a leader?
5/24% = 21
And this still does not account for the fact, that you get 6% to the 4 other planets.
It also doesn't account for the fact, that I can shift leaders to my civs most urgent needs at any time.
If I had known about this in m current game I would not have 3 out of my 25 citizens being leaders. It would be more like 20-ish.
OP is on point on this. Leaders are overpowered as compared to other citizens. Especially early game.
Only thing I can think of which might compensate a little is leveling, leaders don't level, do they? But having 40% instead of 30% after 200 turns is not enough to offset the otherbenefits of leaders.
Hell no, considering how long it takes to get them.
Researching Interstellar Governance? This does not take long. It is verrry convenient to get actually.
Universal Translator you get early, so you can trade with others and lull them with open borders. The specialization you get because you need administrators soon after. And if you got something else for the specialization (whyever you would do that) you get IG for administrators. Your third or so citizen can be your first leader. And then you get nothing else for the next 9 or so citizens.
Does the AI use them like this? Still balanced vs. AI-players?