Kaz Matsui is a peculiar case, he was an ironman in Japan but became injury prone in the majors, very strange. His performance while healthy didn't translate at all from Japan either, except for a month here or there. But he seems to be more of an expception than the rule when you look at players who have transferred NPB to MLB.
But anyways, I recall Kaz having a hot start to his career with a homerun off the very first pitch he ever saw in the majors, and so on. So, I wouldn't get too high on Johjima just yet. Once the scouting kicks in, it would be interesting to see how Joh adapts to pitchers adapting to him.