Heh. I personally wouldn't bother trying to game the rating system by buying or not buying sets or cards...
I would expect that if you do something reasonable, then the good players will float to the top anyway.
Hey! Maybe you can do something crazy like weighting the rankings by card? To guarantee that Colony games count for (exactly the expected percentage of your rating), and so on. That would mean you wouldn't HAVE to buy all the sets to be highly rated, but you probably would have a very uncertain rating with a lot of the cards if you've only played with them like once or something.
It's not a question of floating near the top, but being #1. I wouldn't actually do it - but you know that someone will.
But I don't think it's that big of an advantage.
Like, in automatch, fully half your matches will be with the other people's cards, so that's half the advantage gone. And you don't even pick your own card sets - and Base will always be an option. So the most you could do is bias to Base+1 other set, so you'll get a lot of boring Base games, where you get no advantage because everyone knows how to play Base. I mean, maybe it's a little bit of an advantage because you might beat some better players sometimes in a BM+witch game, but that bites both ways - if you ever get a good ranking playing this way, you'll lose a lot of games you shouldn't.
I just don't see it as a major problem, and I don't see how you could use this to get to #1. Not any more effective than just, like, biasing Prosperity or something.