I was probably playing very suboptimally because my experience with the Yongle Emperor was that he was good but not super OP. I was not fully exploiting his projects to turn my cities into megacities, though, and my largest city when I won (Culture Victory) was ~25 pop, which I actually regard as a little low.
Qin (Unifier) did poorly in my game. He grew Xi'an into a very decent city, but he never founded a second city even though he had plenty of room to expand in every direction except east (until Ba Trieu and I hemmed him in in the late game). Yongle and Wu both did very competently, but neither was really competing with Jayavarman or myself or even Tokugawa. I know there are a lot of variables, and I'd have to see them in more games before I made a confident statement about how the AI handles them--but from my single experience I'd say that, in the AI's hands, they're good but not super. Though China's civ bonus is just a good straightforward bonus for the AI to exploit, regardless of leader ability. (Poor Nader Shah started on a continent by himself so he got to play Persia the way I play Persia: with no LUA.

He was still doing better than poor Qin and Gitarja and at least as well as Seondeok, who was having Genghis Khan problems.)