ORIGINAL: Terminus
At 20 miles range/35 knots speed (your figures, not mine), the torpedoes would take around half an hour to reach their designated targets. Do you perhaps want to consider if said targets might be in an ENTIRELY DIFFERENT ZIP CODE by then?????
for speed/range figures:
http://www.navweaps.com/Weapons/WTJAP_WWII.htm
it's like laying a minefield lasting only a few minutes, the advantage is that you don't need to get very close, but the window of opportunity is narrow to say the least. they had that weapon, all that would have been truely risked in such an encounter would have been the loss a dozen DDs and a couple of CLs (CAs can't be sent, they would probably be viewed as capital ships on radar and trigger a reaction and unpredictable change of course). the potential gain is huge, but it's as unreliable as it can get.
ORIGINAL: pasternakski
This is exactly the conceptual trap that often made the Japanese slaves to their own technology. Just because the danged thing will zip along for miles and miles doesn't mean that you need to abandon study of tactical doctrine that will result in your wonderful weapon being applied to maximum effect.
i don't know if they stopped refining tactics after they had fielded that XXL torpedo. their planning turned out to be completely misguided, though, because the battle they had planned for simply didn't materialize. i hope they designed a weapon for an existing doctrine and not the other way around.
i severely doubt the USN would have played into their hands in any shape or form, the historical campaign was hamstrung by a lack of ships, which would still have been afloat in this scenario. no suicide run to the phillipines, i'm sure. the only real opportunity i can think of would be attacking an invasion task force that way, but these aren't likely to materialize anytime soon after the start of hostilities.
under these cirumstances, fielding type 95 21" torpedos would have been more than sufficient for IJN purposes, obviously. either that or giving the LL a range of 10k yards and a warhead of 2000lbs....