This is just a pet peeve of mine, but thanks to Tom Clancy there is a lot of confusion out there when it comes to "test depth".
Test Depth is the deepest a submarine can dive to
safely. Test Depth is NOT an operational depth, at least not for any usable length of time. Test Depth is calculated by the shipyard and used to verify that all of the sea fittings and exposed seals are working properly. It is only used for new construction sea trails, post overhaul trials, and when the Dive seriously screws up.
In reality, subs spend as little time at Test Depth as they can. Mostly to insure that new systems and repairs work as advertised.
Think of it like your car: Just because the speedometer has a maximum value, are you going to drive that fast all the time?
Instead, think about your HP air reserves. HP air is like gold, you need to spend it wisely. The deeper you go, the more air you need to use to launch things.