Wiki is a good place to start but I don't know about putting much stock in much of it without more research. Primary source, don't think so.
Well, when I read a Wiki article about anything I want to take seriously, I check the citations. You're right that they're not often going to be primary sources, but seeing the claims backed up by good secondary sources puts the article in a different category than one where the citations are from "some dude's website."

In some cases, I have looked at their citation and found that it's a book that I own. I look at the book, and the wiki article almost repeats the book verbatim! Again, looking at the talk page and the history of edits and wrist-slapping that's gone on over the 109 article, it seems to be of good quality. For instance, most websites you look up for the 109K-4 will say it had two 15mm cannons in the cowl and the option of the MK 103, which we know is silly.
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QFT. The gun package is the beauty of this plane and is what keeps it from becoming just another overused pick machine like the Spit16 and N1K - well that and to a lesser degree the less than average views that most new comers to the 109 don't like compared to the Pony, the Spits, N1K, and LaLa. It is perfect the way it is, I admit it would be fun to have the other gun package but I'm afraid it would ruin the reputation of the plane in game and just add another new dimension of horde/pick torture to the game.
The reputation of the 109K-4 in the game is irrelevant. If some 109K-4's had the MG 151/20 in the hub, it should be in the game. If it proves to be too much that option could be a separate perked aircraft.