I don't know how it is in GWX, but in stock SH3, if you go to silent running and the slowest speed they mostly loose you.
If in GWX its so hard, you can try to learn sinking destroyers, which is worth if they are protecting a convoy. Once I had to sink 6 destroyers protecting a big convoy (I was approaching from behind, no time to get around it), then could destroy 2 ships with torpedoes and 2 with deck gun. Not very effective, but I don't like to wait several weeks for another convoy to pass by

. However if I had a IXC instead of VIIB, it would have been totally different and most of convoy would have been sunk.
So in IXC perhaps it pays off more to sink destroyers protecting convoys, unless they are alone, which is a waste of torpedo.