Most Japanese subs were ‘coastal submersibles’ not true submarines. American convoy zones were basically out of reach of Japanese supply for subs, because the pacific was a much larger body of water than the Atlantic.
The Germans were much more systematic in submarine warfare technique because they used the experience of ww1.
In the Pacific the Japanese had the 400 class subs late in the war but they didn’t make many of these and didn’t use them well. but this ‘what if’ thing is a decent idea, but if the Japanese changed the strategy… well they would not be “the Japanese” just like Hitler not attacking the Soviets would not be a real Hitler “type of thing” to do if you get my drift. This does not make it a true ‘what if’ but probably falling outside what was possible in Japanese naval planning
http://en.wikipedia.org/wiki/I-400_class_submarine
and this from:
http://www.combinedfleet.com/ss.htm
Given their size, range, speed, and torpedoes, Japanese submarines achieved surprisingly little. This was because they were mainly employed against warships, which were fast, maneuverable, and well-defended when compared to merchant ships. Japanese naval doctrine was built around the concept of fighting a single decisive battle, as they had done at Tsushima 40 years earlier. They thought of their submarines as scouts, whose main role was to locate, shadow, and attack Allied naval task forces. This approach gave a significant return in 1942 when they sank two fleet carriers, one cruiser, and a few destroyers and other warships, and also damaged two battleships, one fleet carrier (twice), and a cruiser. However, as Allied intelligence, technologies, methods, and numbers improved, the Japanese submarines were never again able to achieve this frequency of success. For this reason, many argue that the Japanese submarine force would have been better used against merchant ships, patrolling Allied shipping lanes instead of lurking outside naval bases. Bagnasco credits the Japanese submarine fleet with sinking 184 merchant ships of 907,000 GRT. This figure is far less than achieved by the Germans (2,840 ships of 14.3 million GRT), the Americans (1,079 ships of 4.65 million tons), and the British (493 ships of 1.52 million tons).