I think the fact that Kara has changed the type of music they release in Japan compared to Korea will be extremely beneficial to them in the long run.
Other groups release either A) Japanese versions of Korean songs, or B) very similar music to what they would release in Korea.
But the reason both A and B are able to do well now is because of the popularity of everything "Korean."
Kara, however, have incorporated a lot of Japanese themes into their releases. Specifically GO GO Summer for it's throwback to 1990's Eurobeat and summer theme, and Winter Love for the pop-ballad aspect and winter theme. They also seem to be moving away from a sexier image to a cute image. In Japan, cute will forever trump sexy.
I believe they will be one of the groups that have the longest amount of success.
But that's just how I feel.