I've been using 100% realism for a few weeks now. I was really frustrated with it in the beginning, but now it's a snap. Practice, practice, practice.
As far as dealing with convoys. I set up all my shots in advance. Just a little something to do while I wait out my 3min 15sec to get course and speed. I set up my torps at the various depths, speeds and salvos (if I use it). I make all of my shots off the bow torps and turn back away from the convoy and let loose with the stern torp as my parting shot. (usually reserved for a close merc that the zig-zag doesn't help very much)
I get away from the convoy as quickly as possible, depending on what the DDs are doing. I set a course to both get me away and to set up meeting the convoy again further down its course. If I need to, I'll sit about 6K+ meters off the convoy and ride the surface if possible to get any external stores inside the boat for later use. Nothing worse than finding a ship to sink and have your torps on the outside!
