Maybe I missed something when I (totally didn't) read the 3 OPs, but I didn't think stats were the main factor in player retention?
Either way, since the topic is up, I would love to see a variety of stats. It would tell so much more about how things went.
For instance...
Damage dealt to balloons
Damage dealt to armor
Damage dealt to hull
Damage dealt to components
Damage dealt by fires lit
Damage taken to *
etc.
This would be much more informative than "I destroyed some things". If you are shooting a gun the entire match, you quickly see where the major damage goes. New players would very quickly learn to shoot balloons with carronades, and armor with gatlings. Because after matches they would see that when they were just spraying, the dealt massive damage to armor. So next time aim at that.
Relating to the pop-up noises, it would be cool to see a point system put in place that does pretty much nothing, but lets you see what you can do.
Below or above your hotbar, have a line where notes pop up.
+10 destroyed enemy armor
+8 destroyed enemy gun
+2 repaired a component
+3 extinguished a fire
+15 destroyed an enemy
-20 destroyed allied (anything)
This would be similar to FPS style games where you get a score as the game progresses. What the score would mean is anybody's guess, but just the pop-ups would be gratifying.
Relating back to player retention though.
How often do people come back to the same game repeatedly for the same experience? Gaming is about experiencing something new. New content -> players come back for a taste. Most games don't hinge on getting you to buy and continue playing the game. Most games hinge on making other people like the game enough to convince you to buy it too. Companies get a good rep for offering new content that keeps players interested.
Indie games are sort of different. Most have bugs and continue to update as they progress. Still, we are in a unique situation where we are wanting to retain players and keep people around to play pvp simply because that is all we have. Most other games either have something else to fall back on, or have a hundred other games so similar that a player doesn't attach to a community, but the style of game. Players hop from game to game following whatever takes their interest at the time.
My grand point here is that as far as I know, the game is so unique and in such a strange place for player retention to even be an issue. I don't really have any solutions, but just consider how many games you played before you got to GoIO, and you never stuck to them like you do to this. Player retention feels like a problem, and may be a problem, but it is only natural, not an epidemic that lurks behind every update.
Just my 2 cents.