I'm not really certain how important the Japanese installed base is to Western developers. I'd have to study Media-Create a bit, but I recall only seeing NDS/WII titles in the top10. Mainstream Western titles don't do too well. Therefore, NA (+Europe) installed base might be more important.You are correct. I was careful not to argue how far Sony will remain behind (could be close, could be far) or the WW dynamic. MS absolutely sucks in Japan. Sony has the ability to completely wipe out, from a WW perspective, any NA gains MS may have by totally throttling MS in Japan. And there is a high degree of probability that will happen.
I find that more likely than MS overtaking Sony in Japan (740K unit difference, only 15K units a month!)
Of course, Japanese titles themselves are interesting in their own right (MGS, FF, BD to some).