The first upgrade is just everything you need , more range, more hp , and AAA. And after that one the second hull upgrade is the cherry on top with even more AAA , faster turning turrets and faster rudder shift. The big hurdle is to get to the first one. But since last major upgrade all JPN BB's have their hull upgrades split into two parts with the first hull upgrade considerably easier to achieve and essentially bringing in all the necessary stats , bells and whistles.