If the convoy is moving towards you and if you can remain quiet waiting it... then why not take the chance?
hours ago I had a convoy with several merchants, 4x DDs and one light battleship (Fiji).
I went into position and remained quiet, waiting them... I took them by surprise, battleship down, 4 large and one medium merchant down from the middle of the convoy.
I was detected just one time, when I surfaced the sub at flank speed to hit a merchant engines (that was half disabled but moving away from me).
Disable their engines, to stop them in the water... (reloading the torps while you launch them) go to cover 70m-150m and stay there for awhile to leave some distance between you and the remaining convoy and its DDs.
once the last DD is from 15 to 25km away... surface and finish the work... kill all the ships that have eng. problems.
This is the tactic that I'm using sucessfully until now, to manage large convoys. Of course would be difficult from 1942/43
Other tactic (kamikaze type) for small/medium convoys with 2 covertts or 2 coverttes + 1 DD or 3 DDs is:
submerged at flank speed with a periscope out to attract DDs/Corvette's attention
- Once the escort around 500m launch the aft torp (set for fast, 1m, impact)... often is a dead DD/corvette.... (if not ... crash-dive+ 90 degrees turn)
- Do the same to other escorts then all convoy is your's to take it down with front torps + deck gun.
Anyway you are the boss of your boat so you are the decision maker
fight or hide is up to you :P