An early German snorkel advantage would invite an early counter

Starting the Second World War with late-war snorkels would make early U-boats more dangerous, but their small numbers would expose the method before Germany could exploit it at scale.

Question

What if Germany began the Second World War with its late-war snorkel system?

Short answer

Early U-boats would be more dangerous, but low numbers would give the Allies time to develop a counter before mass deployment.

Notes

Late-war snorkel technology would improve the lethality and survivability of Germany's early submarines. Germany did not yet possess enough boats to turn that edge into an immediate mass effect. The Allies would therefore encounter the system while its user population remained small and begin developing counters. Advanced technology matters most when the force also has the numbers and doctrine to exploit it.