FYI, all Premium SS are modified SS5s of the same nation with:

faster or equal surface and submerged OH speeds, but if you don't have quality engies, forget it. 

some slightly modified characteristics such as durability and maybe turn ratio or available aircraft ability, and #  and placement of torp tubes(like the MN PSS, for instance).

BO lvl is same as ss5.

TORPERS MUST BE READY for SS5.5 cause all PSS torp tubes are at LVL WITH SS5.5  !!!!
MANY levels ABOVE required BO LVL!!!

PSS is for sure better than SS5s and in some cases, i think better than ss5.5s, especially if engies are a bit  sub-standard(pun intended).

 face it   5.5s pretty much need at least  some 115-120 engies in place, to get surface/submerged speed maximums.  PSS are not quite as restricted due to avg. smaller bulk(max displacement).  IJN is a prime example of the displacement problem in the HUGE ss5 AND SS6...note the supp slots, for pilot(1) sure, but you'll wind up with minimum 4 engies and struggling for speed in both.

 

Thanks for all the responses.

Speed is the only thing I noticed about this ship when I baorded all my crew on it. IJN SS5 and 5.5 are soooo slow unless you have 4 engies in place as you said and you still will be outmaneuvred by SS from other nations. IJN SS4 HEI is better than SS5 in my opinion despite of the shorter submerged time. Guess I will have to wait until I level my torp man to 105 to see how the PSS plays, by that time I should have 4 engies on board!!

Thanks again. 

Seen you in game. Hei is a great ss. I'm currently using the ss5.5 with 5 engies all lvl 122+. Speed is 37knot. Not great but I have lots of oh. 1 Rep on r slot and one TB. 
See you in game!