I wouldn't expect too much out of Japan... I tried a few test games (never played as Japan before), and in three of those I failed horribly when my daimyos revolted and my army was crushed. The fourth time I did better and actually managed to win against my rebelling vassals, although I was heavily in debt. I'm impressed that Rufus managed to unite Japan as quickly as he did in the previous game.
My plan is to spend the first several sessions watching the percentage slowly tick up as I diplo-annex my vassals, while praying to the ancestors that they won't rise up in revolt. At least not until I'm ready to deal with them.
I also don't expect to be alone in Asia for very long. We're bound to lose players in Europe as people expand, and at least some of them will likely end up playing Asian nations in future sessions.