Due to the fact that what are essentially international sales count toward the Oricon, I think only time will tell the true market penetration of korean groups into the japanese market. Pattern may be different for the respective groups since they seem to be marketed in a different manner.
As for the release competition, that's an interesting conundrum. Is it better to be releasing on a week when EVERYONE will be paying attention to the charts and coming in 5th or so or being first or second in a week when no one is looking at the charts...