I am not too sure on that. We know what we have sold to Japan, but we don’t know what kind of improvements Japan has made to the stuff we have sold them, or all about the goodies they have worked up on their own.
Missile defense is a very serious Japanese defense bugaboo. They can also be very good at keeping secrets when they want to be.
It is in their interest to not advertise, but I strongly suspect the Japanese are a lot further along than just using what we sold them and not tweaking things. They are far too good to leave things alone.