This is a rather late reply, but the question addressed still stands on this thread, so I will offer up my two cents.
I recently played Pacific 40 with the new Alpha +3 rules, set up and NO and believe me, Japan can indeed outpace the United States in IPC production, especially if there is real opposition left (ie, China and India) which has been crushed by the weight of air power. The DEI is really all Japan needs, and their fleet to protect it. With China and India crushed, DEI taken, (including the aforementioned NO, but potentially plus the others.) Japan is looking at a potential 70 -80 IPC per turn, compared with the U.S’s 60 -65 IPC per turn.
Perhaps because I was playing with rather inexperienced American player, who very seemed hesitant to attack the Japanese fleet, lest he be left with no operational navy save for a few transports and destroyers back in sea zone 10. Or maybe he wanted to help protect Australia from invasion, wasting time and energy in a defective allied strategy under the wrong circumstances. Or maybe he wanted to build up forces and swing them down south.
Whatever the case, Allied power was disorganized and not brought to bear that game. I want to play the set up again, with more experienced players this weekend, and see how it goes.
However yes, it does seem very hard for the U.S to lay down a truly capable fleet, even with the +5 NO they can conceivably get during the stand alone. Kind of reminds of those “limited war” strategies from the War Plan Orange sessions, one arm tied behind their back.