I can say with GWX I have sunk small merchants with one torp. Admittedly not often - I think 3 all told in 15 patrols. Others, if weather permitting and we are not too close to shore (don't want them radioing for help from shore base) I will surface and use the deck gun.
What I have noticed is that after the torp has hit them their speed will drop. It is those that are unable to get their speed back up to 6 or 7 knots that flounder and eventually go under. So I am assuming some repair work is going on and some are successful others not.
After reading the below link on freighters and tankers that did not sink - one, "Imperial Transport", can you believe, with only its stern left, still managed to survive after been smacked by one torpedo!!!!! I don't have a problem with not finishing off a freighter with one torp.
http://uboat.net/allies/merchants/ships/1471.html
What brought me to this link is that my present boat is U-53 and I was interested to find out what happened to her. She was lost with all hands by depth charges in 1940 in the North Sea off Orkney