Most of the reading I have done indicates that US WWII era subs had their capabilities dramatically understated. The reasons may be partly dues to war secrets and partly due to safety concerns. The US seemed to rate their test depths at 1/2 of the crush depths. Gato's had a test depth of 300 feet and an estimated crush depth of 550-600 feet. Balao's had a test depth of 400 feet (limited to their WWI era bilge pump not working below this depth), and a crush depth of an estimated 900 feet.
There is even a claim here on Subsim that the USS Pollock a Porpoise class submarine with a test depth of 250 feet dove to 540 feet while being attacked. I have not been able to verify this with any other source though.
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