1. Yes, decks awash makes you less visible at medium ranges, also gets the boat under much faster when you dive. At the right depth (25 to 27 feet) it makes you much harder to see while at the same time keeping the diesels running and the batteries charged. As mentioned you can't run that way indefinitely because CO2 builds up below 21 feet.
2. Them hopeless lubbers on the diving planes make 25 to 27 feet impossible without a hack, they need repeated commands and about a week to get the blasted thing down to 25 feet, then they allow it to drift back up to 19 feet and say "close enough".
Because so many other mods use the same files I made a do it yourself tutorial instead of a mod.
http://www.subsim.com/radioroom/showthread.php?t=211613