Thanks for the tip about not using maximum rudder. That helped.
And it looks like the answer to getting a boat to go deeper than test in RFB is - you can't. That's what I thought, but I just wanted to make sure I wasn't overlooking some obscure command.
Just a note, I think I may have been reading something recently here about a Balao reaching 700ft. But of course these measures were only taken in desperation (Deeper chief, deeeper...). This site really does have a fine virtual tour of the Pampanito if you're interested. But just make sure you disable Flashblock first if you're using that addon in Firefox.
http://www.maritime.org/pamphome.htm
I did finish up my first patrol in RFB, and was gloriously successful - winning the Medal of Honor. In fact, a little too successful for my liking. I found a convoy returning home from Northern Luzon to Southern Formosa, and did three end arounds/attacks on those poor nips until I had sunk every single worthwhile target. And through all this I never got so much as a single depth charge dropped, and was only weakly pinged once for a few seconds. This from a convoy with 5 escorts, and many more anti sub capable warships in the normal formation. So I'm back to Trigger Maru for now (my MoH was reduced to a Navy Cross in that mod, heh) and we'll see what happens. I've been tweaking my deck watch so I will hopefully get more warning time with air attacks this time. And if I still really feel I need to puss out - I'll try Webster's 'Better Air Patrols' mod next.
Wish me good hunting, and thanks again for the help.