Read the link I posted earlier. The throw weight of the Japanese 18" gun was virtually the same as the American 16". The maximum range was somewhat longer, but the accuracy would have made long range hits problematical and Japanese shells were known for a high dud percentage. In the final analysis, the Iowa ships would have used their superior speed to close to effective range and sunk the Yamato.
Yeah, both the Yamato class and the Bismarcks are among the most overrated ships of all time (I think Anthony Preston wrote a book devoted solely to overrated and poorly designed ships and had a burning Bismarck on the cover.)
In addition to there really not being as much effective difference in the 16" and 18" guns as people think, there were some serious design flaws in the Yamato armor. I'm quite confident any number of New Jerseys would defeat an equal number of Yamatos most of the time.