Well, you have to be careful about absolute maximums reached in extreme cases vs. operational maximums. In all my reading about WWII U-boats, I've never seen anything about them going deeper than 180-200m (so around 600ft, which was twice their rated depth as it is) as a matter of operational procedure - they only went below as a matter of desperation, or in most cases involuntarily. The actual deepest dive recorded by a crewed WWII-era submarine was by a Balao-class, USS Chopper, (part of) which reached 1,011ft in a diving accident - but I don't think that's something anyone on board would voluntarily repeat!
Because it's a computer game, taking edge cases to set limits is not a good idea, because players would likely abuse these, sometimes unknowingly. Because of the extreme wear that would result, it's unlikely any of these units would actually choose to go to those kinds of limits under operational conditions. It's like with planes - the MiG-25 set a speed record of mach 3.2, but only after being stripped of virtually all equipment, getting structural reinforcement, and still having its engine life reduced to something like 20 hours as a result - so in actual operations, they rarely go above mach 2. Same for subs.
Then there's the purely mechanical side - I don't think in CMANO it makes a major difference how deep you are once you're below the layer. IIRC it does not model the deep sound channel (except with SOSUS sensors, which sort of "fudge" the mechanics to give a plausible representation), so once you're down there, it wouldn't make a huge amount of difference from a game perspective whether you're at 300m or 400m, etc.