Begging your pardon, but aren't the Taigei boats technically SSKs since as far as I know, they use only lithium ion batteries but not AIP?They will increase the sub force to 24 if they build the two more planned Taigei class.
I believe the Soryu and Taigei boats are about the best AIP SS boats built. Very good long range. Good speed submersed. Very stealthy. Very good weApron load out.