Okay, yes those are all pretty big gun packages, but they do require you to put a good burst into the enemy plane to achieve an instant kill, where as a lot of guys, including me (when I flew), are able to get away with quick snapshots by using the 30mm armed birds. With these planes you listed it is generally better to try and "saddle up" on your enemy and wait for a good guns solution. It can take a good 1 second burst of .50cal ammo to take down a plane. So, back to snapshots, while you may not be getting the instant kills you're looking for, you are hopefully inflicting some serious damage that will accumulate to a kill on your next solution.
I don't feel like inundating you with a wall of text, so let's go down the list so to speak.
Obviously the first, and most important factor, is your ability to hit the target in one concentrated spot. Aiming for a wing is probably the best place to put your bullets if they present that profile. If you get a side profile, well, head to tail, aiming for his nose. Since most of your planes use nose mounted guns, convergence is not such an issue, but do try to have them all set at or about the same distance. For the spit, be sure to have convergence set in your general firing range. 300yds is generally what I use for 50cal/20mm.
What distance do you tend to find yourself taking snapshots?
How much time do you generally give yourself when setting up shots?
Do you fire all your guns at once?
Do you know you're hitting with the 20mm and not just the .303's in the Mossie for example?