So, I finally beat the game, so I decided to get the mysterious upgrade. I threw my plain straight up, and held down on the engines, for teh lulz. At about 150 altitude, I turned down into a nose dive. I hit the ground, and hit one of those "Wind up!" things. Then I hit a crane. Then another.
Then suddenly, turtle.
I currently have the 3 remaining expert achievements left (the 3 after Colonel), whatever badge that comes after Upgrade Master VIII, the two after Monkey Maker VI, the one after Score Master IV, and the Send Now badge. :D Fun game. ^^