> At 41 knots and 400 meters the ALFA Class pushed the boundaries of both speed and operating depth.

400 m is a claim on operating depth, not length.

I thought perhaps 400m was their max depth but Wikipedia says 350.

"Max depth" is kind of a nebulous concept though. There's the "test depth" which is as deep as they're willing to test the submarine. There's the maximum operational depth, which is some lesser depth that keeps a safety margin over the test depth. Then there's the "crush depth", which is the depth at which the submarine will actually fail, and is hopefully substantially deeper than the test depth.

.. and along those lines cannot recommend https://en.wikipedia.org/wiki/Das_Boot enough

Been a while since I've watched "Das Boot", but IIRC, there's a scene in the movie where, initially on the surface, they have a taught string strung across the sub. Then, as they dive, deeper and deeper, the taught string relaxes giving a rather frightening visual insight into how much the hull of the boat is compressing.