I am up to 250 secs of OH and same thing, 37 knots is max.
The main problem this SS has is RLD, my torpman is lvl 115 with 4000 skill and 150 vets. It takes too long at this point to reload.
However, because the air time is so great, you do get time to crit dive etc and wait out the big BBs.
I play this SS cautiously. I will head for enemy lines then dive and turn around and let those SSs that *always* head straight for IJN subs come to your HH line. then you have all the time to get to their CVs/BBs.
Doesn't do well on the surface, its weak. Even with 900 SD.
Does huge DMG, its a nice perk. Has enough torps (59) to get the job done every game.
EIDT: 8 front torpedo tubes is excellent, in fact you can be very effective on gradual against other SSs if you are patient. Drrop out of OH and slow to 10 knots, and keep your torps directly at the enemy, don't let them get too close or your toast because you can't proxy like KM and you can't turn fast or fire out the rear.
conclusion: if you have an excellent elite torper and hit the reload cap, this thing is an elite killer. 7 salvos of 8 torps is massive damage. huge splash radius and good damage means your 31 knots OH underwater still won't let a bb6 get away unless you have to crit dive often.
basically it is the patient version of the SS4, and it has the airtime and 5 engineer slots to make it work, and go further as the SS4 has limited torps/airtime
