A quick post on the myth of Test Depth.

[ET2SN]

In the game, Test Depth is used as a normal tactic. Just consider that as you dive deeper, the water pressure increases. The water pressure not only acts against the hull, but also all of the piping systems, seals, and pumps that move it around. Since the game doesn't model sea pressure as more than a binary switch, the tactic stays valid.

Imagine you're buying a new car. You do your homework and find the model you want can get to 110 MPH. Once you buy it, are you going to try to drive from New York to LA and back at 110 MPH? Wouldn't that be foolish? To keep the metaphor going, its highly doubtful you would ever get close to commanding a ship if you had 40 speeding tickets on your record.

In reality, Test Depth is used to validate the ship as a whole. You go to Test Depth after an over-haul or heavy maint. to make sure everything works as rated.
In a combat situation, Test Depth really works against you. Primarily, it takes more high pressure air to launch anything out of the tubes as you go deeper.
Instead, rely on your cavitiation curve. If you can't go faster than flank, what point is there to going deeper once you're no longer making bubbles?

One last point to ponder-
The term Emergency Deep (150 feet) is based on two factors:
-Its deep enough to avoid a collision with surface ships when your ship control might be marginal.
- You can safely pressurize the fire fighting piping and hoses without relying on the trim or drain pump. Going deeper could mean rupturing a hose or getting your damage control teams air-borne.