This early in the war, I'd go in surfaced with decks awash(7m), probably no faster than ahead 1/3. Just make sure you are ahead of the convoy before you start making your attack run. Don't get greedy though. Get in, pick your targets, fire, and get out. Rinse and repeat. You keep the initiative when you can remain on the surface. Dive and you lose that initiative. If sucessful, you can probably make two or three attacks that night.
Probably the most satisfying feeling in SH3 is a sucessful night surface attack when you are creeping away on the surface with a couple of ships burning behind you and the escorts are off looking for you and firing starshells in the wrong area.
Enjoy the feast while you can.

Late '42 and on, it becomes very difficult to attack on the surface when there are more and better escorts equipped with radar. By '43, it feels as if surface attacks are next to impossible against convoys.