I'm playing devil's advocate here, but at the upper limit of a really fast science game, you're limiting total population by sending all trade routes to the capital, right? I mean, it gives diminishing returns. I don't think anyone would disagree that you get more total population by spreading those trade routes out, and more "bang for your buck" sending them to your most recently planted cities, because getting their hammers & gold up quickly is quite valuable.
So, obviously, Monarchy (and the happiness associated with it) and the +50% from NC (assuming you put it in the capital) is the primary advantage. But, if your population is growing slower, then it diminishes the value of that +50%. So, really the advantage is that you need less total happiness. But there are additional disadvantages. #1, if your other cities are bigger, they can build things faster, so you don't end up with 3 turn public school build times in the capital and 20 turns everywhere else.
Obviously, there's bonus growth from Tradition as well. But if you're limiting your growth by sending all trade routes to the capital, that diminishes as well. So, really, if you can find a way to solve your happiness issues, liberty has some advantages... Specifically, in the case of Korea and Babylon, their UA gives more value the more cities you have... it's not based on population.
I'm not a huge fan of liberty for SV, but I thought I'd throw that in to the mix as food for thought. I generally struggle with happiness, except for those occasional games where all the CS quests just go your way and the mercantile civs want the iron you traded away, and your archer is right there when they need a barb camp... ie luck. When luck goes my way, with Tradition sometimes I'm sitting at +17 happiness wishing I'd sent my trade routes to my other cities. Still favoring Tradition, but in a game where you get lucky with lux trades and CS, Liberty might be better... except for that faith GE thing. There are really diminishing returns on faith GS. That last faith GS buys like 3 faith GE, which can finish hubble + 2 parts, or big ben to buy labs cheaper, etc. etc., Notre Dame to fix happiness... if you know you have huge faith and are going to have extra that doesn't equate to another GS, this can really come in handy. But, the per-city science bonuses of Korea and Babylon make me wonder if maybe 6-city liberty has an advantage... less turns to achieve the same population levels, more bonus beakers per population.
Thoughts?