Type XXI: A Game changer?

[Gargamel]

I am currently playing a late war career using a type XXI. After my first patrol got completed, I was astounded. This thing is amazing. 20 Knots submerged? 6 forward tubes? Snorkel? A small pack of these, like 4, would have decimated, or even completely destroyed an early war convoy. But I got to thinking, you really can't play the what-if game. But I kept thinking that if the Germans had the XXI's earlier in the war, the outcome may have been much different.

Then I bought U-boat war today (via subsim link!), and I quote Blair, from the forward:

Prominent experts gushed that the Type XXI represented a giant leap in submarine technology, bringing mankind very close to a "true submersible." Some naval historians asserted that if the Germans had produced the Type XXI submarine one year earlier they almost certainly could have won the "Battle of the Atlantic" and thereby indefinitely delayed Overlord

How much affect do you think these super boats would have had on the war if they had them even earlier than Blair proposes? Assuming of course the obvious faults found in the XXI were either properly designed to start with or fixed soon after. How much would this sub have mattered without the late war pressure on it's production?