I tested a little last night with this mod on. I hit a tanker (large) and she kept steaming. The convoy as a whole did the "mill about" nonsense, but for my second spread (aft tubes) I got a speed of 6 knots on her (quick set up, not sure how accurate, but 2 fish hit, one was a dud).
The escort held me down, then a DD showed up (a subhunter patrol near Truk) to help him. When they finally left 30 DCs or so later, the tanker was dead in the water listing to starboard and settling by the stern. After a couple hours I got the ship destroyed mnote, but she was still wallowing, she eventually capsized. VERY cool.
tater