Japan actually won on turn 5
Well yeah, that is what happens when usa builds stuff in the atlantic when Japan is snatching up NOs. Like you said the -38 does not kick in till round 4 when you are parked off east usa. Plus it is not -38, because you gotta roll your convoy damage and very rarely does that max out.
Sure USA makes -38 or whatever, but from a big picture of the long run… it is really insignificant when Japan flops out.