Not sure how I missed this Fangraph piece by Patrick Newman. The most interesting thing that PN and others note is that Japanese pitchers with small hands can't throw the forkball (split or split variation) stateside as the NPB uses a smaller ball.
Newman notes how critical that pitch is for Iwakuma as it is his best pitch, with a 19% (!) swinging strike rate. If he can't throw that pitch, he will have problems to be sure. Also note that in contrast to other profiles, his take on the data shows that Iwakuma doesn't rely on a cutter or a shutto all that often after all. However, given the questions surrounding the accuracy of the pitch data(in addition to a possible confusion by some US commentators between a cutter, a shuuto and a changeup), we'll just have to wait and see which profile turns out to be closer to the truth. It was said by some that Colby Lewis threw a shuuto that he learned in Japan but that proved not to be the case, as he learned a traditional cutter and altered his pitching mechanics _before_ he pitched overseas.
Anyways, per Newman, Iwakuma's pitch frequency breaks down like this: