So that means assuming I can create a big colonial empire, it pays off to be Mercantile then? Ok
Apart from that, I was wondering, as Portugal, if most of my provinces are overseas and cored, would it make sense to move my capital to the Americas?
And what do I do in the first 40 years? So far I've been going with the "hide in a corner, build up the treasury for colonization" approach but it sounds quite boring ^^
Mercantilism can be good, and they apparently improved it a little in the beta patch, if that should be played. Still, the mercantile strategy is quite long-term, relying on the accumulation of territories and bonuses that are quite distant in the future. Free Trade might look like a bother when you see that it will take the better part of a century to move the slider to the more beneficial position, but even if it is not +5, it will still allow you to compete in foreign CoTs. Expect that England, France, Brandeburg and Scandinavia will move to free trade regardless of their initially mercantile sliders.
As for mercantile, it will mainly pay off when you reach the "monopoly stage" in mid-game. At that point, you can compete out everyone else and reap the full benefit of the CoT as the sole trader. However, until you have a monopoly, mercantilism mainly assures that you will be among the four top traders in the home CoTs, since having a good compete chance does not remove the free trader countries that also use your CoTs (though you will gain a slightly higher income). Ergo, you will have ensured access to your CoT, but the Free Traders might have ensured access to tens foreign ones -- until the monopoly sometimes forces them out. Still, the mercantile bonus may help to conquer CoTs and have a high infamy score, while being able to trade in them. This has to do with the fact that a CoTs in an uncored province damages you over-all compete-chance, often forcing Free Traders out of their other CoTs -- at least in MP where there are a lot of Free Traders.
The debate over Mercantilism and Free Trade rages on, but it would be my personal suggestion to you as a MP beginner to go Free Trade, simply for the fact that it is easier to understand. You will not lose control over your own CoTs and you may gain access to new ones. As for the "big american empire"-idea I see as a prerequisite in your mercantile consideration; do not assume you will get it, or even that you will be among the four first to gain "Quest for the new World." A little modesty has never hurt anyone.
Apart from that, I was wondering, as Portugal, if most of my provinces are overseas and cored, would it make sense to move my capital to the Americas?
If your tariff income vastly exceeds your tax and production income and the potential decision bonuses does not make up for it, then yes. It's not something to do in a rush, but when it becomes possible, think about it.
And what do I do in the first 40 years? So far I've been going with the "hide in a corner, build up the treasury for colonization" approach but it sounds quite boring ^^
You don't do much. If the MP game has one of it's Dawn Wars, then you might have some fun intervening (on the side of Castille perhaps). If you want to take a small chunk out of Morocco, but there are pros and cons to that. Charge cavalry! And heathen low income region that slows down tech progression a little...