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.

I agree HEI was much better and I am ging back. 

Any idea why am I getting this message when I click on the TB slot on the ship

(this option is not currently available) ?????

This Thread might be outdated, but still I would give a comment:

1. The invalid option message regarding loading TBs in the harbour probably is caused due a lacking TB pilot of a sufficiant level in the waiting-list.

2. IJN SS5 and SS6 have more support slots than most other nations and, as you remarked, more air supply and more torp-damage than IJN SS4.

They are not supposed to be sub-hunters, except using their possible TBs for distant sub fighting.

With a decent crew (high leveled and BVE) they are deadly major ships killers.

With my SS crew > lvl 120, I never ever went back even a single time from the moment I got the SS5 like about 6 years ago.

For different purpose I also have other SS-lines from other nations.

What is the max speed for IJN SS5 and SS6? 39knots?