I know the single 20mm would seem hard to kill with, but it turns out not to be that bad. One of the reasons, I'm convinced, is that it's mounted on the centerline, so it's a lot easier to get the hits where you want them, in the critical areas. It's a lot easier to kill with the single 20mm in the nose of the 109-F4 than, say, one 20mm in the wing of a Spitfire (if the other was shot out, and not including the effect of the other machine guns).
Also, you should practice ammo conservation. I think the problem is your familiar rides all have lots of .50 caliber machine guns with tons of ammo, and you can get away just spraying it out there and walking them in. With the single 20mm in the F4, or the single 30mm in the G14 or K4, you can't get away with this. You've got to aim more carefully and fire very short bursts of 1-3 rounds each. You can't really walk them in. It's a different mindset. I have had the opposite problem in the past, where I was so used to firing very short bursts with the K4 that I'd get in a Pony or Hellcat and fire very short machine gun bursts and not kill the enemy even with a good hit.
