The Soviets certainly played a huge role in the defeat of Germany. That said, a huge amount of war materiel used by the Soviets came from... the USA. Almost 60% of their highly refined petroleum products came from the US, for example (can you say "aviation gas?").
Soviet advances would have been far less easy under a virtually unopposed Luftwaffe, even giving "General Winter" his due.
It would be interesting to imagine the vast majority of U-Boats attempting to exclusively interdict tankers for Russia, keeping just enough boats elsewhere so that the allies could not put all ASW assets North. Of course they'd need to have made such a plan, then they'd have needed to shut the hell up about it, and tell the subs to stop using their radios like teenage girls on the phone
Regarding the sub war vs the IJN, this is also important. German subs did effectively no damage to Allied
naval capability at all. USN subs, while practicing "unrestricted" warfare, also sank or heavily damaged many significant warships—warships were critical to the IJN's (completely wrong-headed) Mahanian strategy for winning a "decisive battle."