I have SHIII with no supermods (I have some cosmetic mods like the ship explosion mod, etc). I decided to get U-flak with three quads. Before, I had trouble damaging planes, and generally if I did shoot down a plane, they inflicted far too much damage/casualties to make it worth it. But now with my u-boat bristling with AA guns, I figured I'd even the score.
I decided to do a 'test patrol' by sailing near england leaving my radar on, to lure in planes. I was pretty impressed with how many planes I could shoot down, but again the victory was costly- the conning tower puts so many men abovedecks that one unlucky bomb hit takes out a lot of crew members
However, I still intend to keep the U-flak, because it is certainly devastating against a small number of planes; the clip size and amount of lead in the air certainly rips apart approching planes.