Its a well known fact that it wasn't technological inferiority that killed Germany. The true killer was her industrial inferiority compared to the Allies. The XXI at the time it was introduced in the dying days of the war was capable of defeating all the advanced escort tactics and detection systems. The XXI was a huge step forward as it was a U-Boat that was actally designed to operate underwater primarily while all previous U-boats were merely surface ships with an ability to submerge as a means of escape.
You can look at the XXI alone and know that it could have owned the shipping routes throughout the whole war but that is a very isolated and unrealistic way to look at it. If there was a way for the XXI to have been a major impact on the Battle of the Atlantic it would have required more than just initiative to get it produced. Under the circumstances where there could have been enough type XXIs early enough in the war Germany would have to have had a much better industrial output and of course if this were true then the whole war would have been affected and thats much bigger than the XXI.
So in reality all these what ifs taht involves more than just individual decisions are more than just small little considerations but really involve re-evaluating the whole war.
EDIT. and yeah STEED is right. The Americans had the bomb so Germany would have been blasted anyway. Germany's nuclear program was a dead end. Though as I said if Germany could have produced the XXI in 1940 in large numbers it cuold have changed the way the war went completely because more production increases Germany's ability to fight Britain or maybe invade it successfully or maybe not lose Africa or maybe actually be able to overrun Russia before the Winter.
These waht ifs involve changing alot more than just the Nazi building qeue.
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