I voted for early war because SH4 is probably best within its limits if used to simulate the converted Fleet Boats (Fleet schnorkel, Guppy...) rather than later ones. I am thinking specially about electronics, sound travel through water and so on, specially the cascade display of sonar (Late 70s onwards IIRC).
Sub Command covers well later periods with SCU/SCX

but the 50s and 60s are somehow a forgotten area that would be well served with a full SH4 conversion (Fleet subs to SSKs and German subs to Soviet subs)
Good luck with the project guys