(I run GWX and SH3 Commander, no other mods). I save my flak gun ammo for trawlers. Sometimes for amusement on cargo ships. Never for planes. The only things I do when I spot a plane are:

1) Scream "Alaaaaarm!" in my worst fake German accent;
2) Hit the "C" key for crash dive;
3) Wait until submerged then knuckle to port;
4) Occasionally pop the hull on the bottom when I forget that I'm in less than 70m of water. (Then load Bernard into a vacant torpedo tube for "his" incompetence.)

I barely passed the flak gun Academy mission - I had problems taking out those ridiculous biplanes. No way I'm taking on a pair of Hurricanes.

My IXB is beautiful - truly beautiful. No holes in the IXB, thank you very much.

worst thing is when you spot a convoy and you head in their direction you get fairly close to them on the surface but then have to dive coz you get "airplane spotted" and cant catch up with the convoy. then coz the planes have seen you all the damn dd`s come over to investigate.