One big thing to consider is your research multipliers. If you are decking out your capitol with Libary, Uni, Observatory, Academy, and Oxford as you say, then it will have a +225% research modifier. Your other cities probably have 25-50% unless you waste a bunch of hammers in all of them.
In a late game scenario where your capitol is really well developed, you will probably have a net loss of something in the neighborhood of 20-25 commerce in the cap by switching to FS. If 20, then 20 commerce * 3.25 = 65 beakers lost. 20-25 commerce lost is just a ballpark figure, but it's really easy to calculate in your own game. Bureau commerce bonus - (towns worked in cap *2).
Now if we are generous and say that you have Library + Uni in every other city in your empire that is working at least one town, you would need to generate at least 44 more commerce to see a net gain in total research. That would require 22+ towns in this example. If you've done a good bit of conquering you will probably have that because the AI likes to build them everywhere and it makes sense to keep some of the more developed ones.
The other factor i left out above would be civic upkeep cost because Bureau is definitely more expensive. That may actually translate directly into beakers by being able to switch some Wealth builds to Research. If you have a large empire that will be pretty significant. And there's also the hammer loss in the capitol, so it is a pretty complicated decision. I would imagine large empires with like 20-30 cities will favor FS, where smaller ones will be better with Buruea, but if you are really curious you should probably just save the game, switch, and evaluate the difference.
*I think all of that is right, but I'm tired