I don't think Ryu/Ken is going to be that much stronger than just plain Ryu. Ryu is kind of a stand-in for everything that represents fighting games to begin with. Ryu/Ken is definitely an iconic rivalry, yeah, but I can only see them boosting so much.
Red/Blue is very different. That rivalry represents a ton of good feelings people have about RBY. Just Red or just Blue is one thing, but that rivalry is one of the most famous in gaming, probably more famous than Ryu/Ken. Red/Blue has a good chance of being closer to RBY than it is to Red. Will that be enough to beat Ryu/Ken? Probably not, but it isn't a terrible pick.
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Black Turtle did a pretty good job.