How much would competent Japanese ASW change the submarine war?

Could American concentration of submarines overwhelm even an effective Japanese anti-submarine force?

Probably, but competent Japanese ASW would have made the campaign substantially longer and more difficult than it was historically.

American mass could eventually overwhelm even an excellent Japanese anti-submarine system. Japan's historical effort was so poor, however, that merely average escorts, doctrine, and patrol practice would have preserved far more shipping and slowed the campaign. Competent defences would raise the time and cost required for the United States to exploit its greater capacity. They would delay the outcome even if they could not prevent it.