Well, I'm really not sure where this goes, but since the only mod I'm using is GFO, I guess I'll put it here. I've been getting back into SH4 and I started an American career in a Tambor class sub. I upgraded to a 2x20mm AA gun for my second patrol. Now I don't know if there is the same problem with the single 20mm AA gun, but I had a need to man the gun in the second patrol, and when testing out my field of fire, I noticed that if turn let's say 70º or more to either side, my vision is blocked by the walls of the sub. I'm not sure if this is something that can fixed in a mod or not, but if it is, it certainly falls into the category of GFO.
And I've also found that I can't use the hydrophone in the Tambor or the Gato class subs, but I could in the Porpoise. It's not a deal killer, but it would be nice to be able to manually work it like I can in the Porpoise class sub.
Other than that, I'm having an enjoyable time getting back into the game.