Making teams of fixed size would make the whole thing more interesting, with less drama, as it becomes more of a test of skills than an effort to simply swarm the defender, an effort that in the past has been accompanied by attempts outside of the op wars themselves to get people to quit playing so as to weaken the opposing faction to the point they can't put up enough people to win the next war.
Even though this game isn't a true sandbox game, artificial restrictions are never a good thing. Particularly since you'd still have the "meta game" that you describe happening behind the scenes. (Disclaimer: I'm a member of the faction that is currently on the losing end of "the swarm").
As long as the outposts have something that benefits just about every facet of the game, selfishness and greed will bubble up to the surface and define how people interact with one another. It boggles my mind, and still stings a little, that people will throw away friendships and bonds and unity because of some pixels in a game.
I still maintain that to keep PvP and still bring things back to the "fun" level, remove the benefits from the outposts and bring them back in a way that not only makes sense within the game (digging for cats?) but also depends only on player skill within the game, not behind the scenes.