YOR can tax at 100% with no penalty !

It seems unrealistic if they don't can that. I didn't play with yor until now but against them it is not more or less difficult as against other species .. 

Since the the changelog says you can it reasonable to conclude ai will.

The balancing downside is that it takes an excruciatingly long time to manually build their population up. 

The Yor haven't lasted long in my last few games.  In fact,  I've had to prop them up with tech, etc to keep the other races busy.

The Yor don't seem to have market buildings -- at least,  I didn't notice any in my first few stabs at playing the Yor.   So I imagine this is working as intended and hopefully reasonably balanced.

To answer the OP - I don't think so (it certainly doesn't feel OP),  and I hope so.  

For me too. Its tempting to have 100% taxed population, but they are not that strong. However, if you do get developed, the machines CAN take over the galaxy, there is that potential.

Which is ironic considering it should take much less time for a machine to build another machine than to organically grow a person.

But on topic, no, the Yor is terribly weak now in the hands of the AI. They are almost always among the first destroyed, and in less than 100 turns on higher difficulties. As a matter of fact, minor races last longer than the Yor. lol

Seriously, they are always in last place on the power ranking, with the smallest territories and number of planets. Even the factions that I play have a higher power rating than the Yor, and I am the faction always sitting at a measly 300-400 while everyone else is usually at 1500+ because I do not bother building a fleet until almost turn 100 (thank goodness for that one Pragmatic Ideology).

I miss the days when the Yor were to be feared because they would almost always be among the top with hundreds of ships gunning for me (only to be destroyed, but that happens to everyone that attacks me), unless they got a totally crappy starting point. Oh how the mighty have fallen.