I would like to tell al little story of my own sub campaing.
I am playing as Germany on hard/furious with DAIM installed. Furthermore I have also set the houserules of not building any normal infantry, brigade everything, use both fighters and interceptors and not to do sealion untill after I have invaded the sovjet.
Since I had to use quite alot of IC on mobile troops, brigades and other nice stuff to help me fight a successful land- and air- war i could not (and did not want to) build a large fleet. All in all I was trying to recreate the real WW2, but still trying to win

.
I have found that the four, I repeat only FOUR, sea areas between portugal and greenland is what you need to patrol. I used 2x3 subs pack in each area mounting up to enormous 24 subs needed. I had 36 at the start of the war so I could keep a few in spare and also patrol around the Azores, a great hunting ground for convoys.
After only four months of convoy hunting (I put the subs on hunting both day and night) I have reduced the british convoy numbers from 1050(including new builds) down to 570. Similair numbers for the escorts: from 180 down to 60 left. I have lost 4 subs and damage a few DDs and the occasional transport. I always retreated as fast as I could.
My sub numbers are now up to 48 so I will start convoy raiding in the areas between south america and africa soon, creating two lines of interception of any convoy(gibraltar will fall along with rep. spains soon).
The british trading efficiency with the US is down to 52%.
Only strange and slightly annoying thing is that UK has not started building any large numbers of convoys or ship to try to hunt my subs yet, perhaps sinking half of their merchant fleet in 4 months was to fast for them to react, perhaps it is DAIM.
My conclusions: Subs are great and you dont need as many as 60 of them to wage a successful war with them, infact 24 + a few spare is more than enough at the start of the war, even on hard difficulty. As you build more subs you can extend your hunting grounds and sink even more convoys.