Bringing this back to the actual topic I think that a 6.5% hit rate is actually not at all inconsistent with IJN doctrine.
The problem is that the 6.5% (ish, I got a slightly lower number when I tried to replicate the result) is not consistent across battles. It's only THAT HIGH because there are several engagements with extremely high hit rates. These outliers consistently occurred when the USN was not aware that they'd been detected. They're very useful outliers because they tell you alot about the circumstances in which a torpedo attack can be really devastating. You have to detect the enemy within reasonable torpedo range (less than 8.000 yards-ish for the IJN, 6,000 for the USN), with sufficient time for a firing solution, and with sufficient more lead time for your torps to be in the water BEFORE the enemy starts to maneuver as a reaction to your existence on the battlefield.
The modal hit rate (the mode is the most frequent outcome) is zero hits and the median (if you splut the distribution into two halves, the one in the middle of the distribution is the median) is also zero hits. So unless that Decisive Battle occurs at night at less than (generously adding 25%) 10,000 yards against a USN TF that doesn't know the Japanese are there (or doesn't think they've been detected) the decisive battle isn't going to be a success.
The second thing to note is that after Tassafaronga (which was generally a tactical fubar on the US adm's part) and in some instances before it became standard practice for US ships to maneuver to avoid presumed torpedo launches when engaging the Japanese. So the necessary conditions (night, surprise, in torpedo range) were only likely to occur a couple of times anyhow, before the US "catches on." And it's only likely in 1942, because US radar improvements made it most likely that the USN would always know that the Japanese were closing to torpedo range after 1942 (excepting Japanese subs running submerged).
given their purposes a 6.5% hit rate from long range actually achieved their purposes
They're not going to get 6.5% from long range. They're only going to get 6.5% ON AVERAGE if they surprise the USN in a couple of battles at ranges less than 10K yards. At long range their average will be alot lower. And the modal hit rate will still be zero. That is, in most battles, all torpedoes that the Japanese launch will all miss.
GREEN BUTTON ME AGAIN, QUICK!!!