I thought I had read somewhere that the Germans would alternate ammo types in their cannon belts. Something like 1 in 3 or 1 in 5 of the 30mm rounds were actually the devastating HE Mine rounds. The others were AP, regular HE, tracer or other types. It was said that the pilots would actually keep count of the rounds they fired so they would know what type of round was the next to fire, and cycle to the round they really needed if necessary.
AH did not model the various different round types, but compromised by taking the average destructive capabilities of the types of shells used, so we end up in AH with a HE round that is a little less destructive than a HE Mine round. This might explain why the damage is less than anticipated. Granted, several 30mm hits of even very low explosive capabilities should do quite a bit of damage to an airplane, but maybe this can help frame the discussion a little.