Supercharger switching is automatic in the P-51. As you climb or descend from very high, watch the manifold pressure gauge; somewhere around 4000m-5000m you'll see it jump, that's the supercharger changing stage.
I don't think it's possible to get smooth panning with the joystick hat, but maybe someone else here knows how to do this via changes to the config document. At some point I had smooth panning by using "joytokey" (
http://joytokey.net/en/), which I set up to make the hat emulate the mouse. It took a few minutes to set up but it was definitely worth it until I switched to the mouse entirely for looking around.