If all of your guns are set to IP at 500, the only difference will be time to target. If your barrel is set level with the ground with all guns, your IP at 500 will be different. Sit in say a 109F offline with all of your guns set to 150. Pull up the offline target and set it to 400 with an azimuth to lift it up. Lets say you spawn north east.
.target 500 45 12
500 yards, heading of 45 and pitch up of 12 degrees.
First fire the 7mm MG, then the 20mm motor canon and see the difference in IP points.