Wouldn't you say that getting a very high hit percentage is a matter of discipline than skill? When I ask this question I am not really referring to the buff hunters or one hit wonders. If I wait til I can see the whites in their eyes its not really an issue. However when that 109 is out running my 4 fifties (P-51B is my plane of choice as of late) beyond 600 yards and I really do not have time to wait him out.
I think the answer to this is, if you can maintain a very high hit percentage, and still keep your kills per opportunity at the same high rate then it definately is skill. I know I kill my hit percentage exactly for the reason you describe, if someones getting away I'm not concerned so much about wasting ammo as I am converting the opportunity to a kill.
If a pilot gives up kills others would rack up for the sake of retaining hit percentage then it's entirely dependent on whether or not they make up for those kills by using the ammo on other targets before returning home. In Skyrocks case I'll give him the benefit of the doubt, he tends to fly in high concentrations of enemy planes, if he avoids getting shot down on any given sortie, ammo saved not taking low percentage shots more often then not will be used successfully in high percentage situations.
I'm more aircraft dependent, if I'm in a K4, the taters definately get conserved. In a .50 plane I'm far more likely to spray a runner down and to hell with efficiency.