Erich Topp in an interview about the type XXI states that in his experience, the type XXI would NOT have made that much of a difference. Yeah the amount of tonnage sunk might have increased, but the problem with the Type XXI was that it was designed to operate solely underwater more than on the surface. And doctrine for the type XXI apparently, according to Topp, who did command such a boat, that shortly after leaving port, the boat was to submerge, and then remain so to avoid detection. But, that may only have been a result of the heavy Allied air presence late in the war.
And Eric Topp states that this would have made forming wolfpacks against convoys difficult as when they are running submerged the subs are out of radio communication with base. The resources the Allies were able to pool against the germans in terms of ships and ASW efforts was just too much for the germans, even if they did have the type XXI earlier.
Infact, the interview with Topp is on uboat.net's website
