In my opinion, HEI I is the climax of ijn ss.
Kai Kou and I 400 can Launch TB and it's the attraction for player. But the 2 type ss can't prepare TB while under water.
Kai Kou get air maintenance cap (in my test at 1 year ago) and I 400 with same air maintenance. No air maintence advantage against other nation ss6 (they get cap too)
Then the speed ,turning ability and torp relaod time even get worse(51.5sec for Kai Kou , 60sec for I 400. Even topedoman get cap , you still need half of time to reload.). If you can't slow down target at 1 shot , you will be hard to get 2nd chance because of lower speed and reload time.