You have the sub the vast majority of sub skippers wanted! As Sam Dealey said when he was asked where he wanted his deck gun mounted, "I don't care whether it's fore or aft, just so it has wheels so I can roll it overboard." He figured the extra knot or so of underwater speed was worth a lot more than a useless pop gun whose main function was to lure subs into getting their fool selves killed.
That all begs the question: is the sub in SH4 faster underwater without the draggy deck gun? Or is the deck gun just eye candy having no bearing on anything? My money's on eye candy......