I think we're agreed that we don't have a definitive answer, and may never. The only thing I'm still insisting on at this point is the trials. For surface ships they really did pull out all the stops, running with no ammunition, no stores, minimum crew and fuel, and gave it everything they had. In the early days of destroyers (British, 1892), when they were coal-fired, they actually doubled the shovellers, working them in two shifts, ten minutes at a time, so they were shovelling their absolute hardest all the time.
Unfortunately I haven't been able to find any official trial data for any submarines. I suspect they didn't run speed trials because nobody cared if they could reach their designed speeds. After all, no sub is going to outrun a destroyer (pre-nuke of course).
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