I've taken a type VIIC down to around 220 and had no issues. That was with no damage though. I've taken my IXC recently down to a test depth of 240 and not sustained any damage. But I dared go no deeper.
I think that you can go as deep as you like. Just be wary once you get blow 220. Usually though the crew will tell you "we're taking damage sir". And if you aren't being DCed at that moment then you know its the depth.
__________________


|