Theoretically it could flex more but at a certain point you couldn't aim down the way it was pointed.
You'll find that Aces High simplifies such gimbles often by giving the gun position a fixed mount but with a wider range of movement than that fixed mount would have to compensate for the likely range of motion. For example: The He111 gondola guns and the dorsal position.
This is to reduce complexity. I believe HT or Skuzzy has said before they COULD make it very complicated giving one set of controls to turn/rotate the run, one set to lift/raise the second ring, a third set to slide a gun position around the second ring, and then mouse/joystick to actually aim it there. The general response was that was super complicated, time consuming, and nobody would use it. They simplified the graphics to get the same practical effect.