I think it's a matter of perception. In Il-2 the damage was almost always spectacular and exaggerated. In CoD, the damage effects are more subtle and realistic. For example, the other day I got hit by a short although accurate burst by a Spit, the only thing the Spit pilot noticed was a tiny vapor trail while I had dozen or so systems knocked out or damaged. Now I would not be surprised if he felt that .303 were ineffective, however that was not the case.
Same thing the other way around, most of the time it's not obvious just how much you damaged your target, most of the time the enemy seems to lose control, slows down or exhibits the signs of reduced maneuverability with very little visual cue on the damage inflicted (although with external views the damage gets more obvious).
I agree that at the moment cannons do seem to be under performing compared to machine guns. As if 20mm API shells do not have a significantly more chance to set something on fire (compared to MGs) while HE shells seem to work well only on control surfaces.
Anyway it is difficult to test those assumptions, perhaps in the future when we get the SDK we could build a test rig where you'd place an aircraft on the ground and shoot at it in a controlled manner to test the effects
