I think that the question is interesting, but the main conclusion I come to is that the munition that the plane carries is moot. The .50s and the 20mm don't have wildly differing capabilities as modeled in game. In capable hands, they will wreck soft ground targets. Perhaps, if there are no enemy fighters, the P-47 is slightly better because of its ammunition count. Perhaps if there are enemies present the Tempest is better able to defend itself air to air. But these factors are so subtle and random, they are impossible to plan for.