Keep in mind that ideal periscope depth will vary with sea state. I suspect that periscope depth in game is good for having the scope just clear a calm sea surface with a good view and not too much exposure, but still have the sub as far down as possible. As sea state varies, you may have to adjust this depth. This would explain why people are reporting variable experiences with the preset periscope depth. I'll test this one.
I believe you are correct with regards to sea state and maximum concealment. Secondly, you can extend your periscope farther than it normaly does by holding down CTRL and the scope raise button.
I've also noticed the sonar seems to work better a few feet below the default periscope depth. I think this may be a bug, but diving 5 meters or so below default scope depth seems to allow sonar to function correctly. Using CTRL you can still raise the scope high enough even at the lower depth.
I'd like to recommend a few mods, first up is Captain Midnight's CBS radio mod. It adds a new radio station that plays period music, then the music dims and news recordings are played as historical events occur in game.
http://www.subsim.com/radioroom/showthread.php?t=108756And I like the Fairbanks diesels,
http://www.subsim.com/radioroom/showthread.php?t=110470When I get home I'm gonna try this one,
http://www.subsim.com/radioroom/showthread.php?t=108512