When it comes to the representation of AA guns in the mod there are several different topics to cover, including historical effectiveness and game engine limitations.
As for the historical effectiveness of German heavy AA guns in WW2, this is from quora:
Effectiveness - Based on WW2 German figures, in 1944, the 88mm flak 36 (most commonly deployed model) anti aircraft gun required an average of 16,000 shells to bring down one aircraft flying at high altitude. That works out at 117 Tonnes of Ammunition per aircraft shot down which is the same weight as 50 Messerschmitt Bf 109s Fighter Planes. British AA guns expended similar amounts of ammunition per aircraft downed during “the Blitz” 1940–41.
Numbers of Aircraft - During WW2 the Germans credited their Anti Aircraft gunners with the destruction of 2,598 aircraft out of 14,938 (17%).
Technology - German AA effectiveness did not show any significant improvements during WW2. The USSBS calculated that a working proximity fuze, similar to the ones deployed by the Allies from 1942, would have increased Flak Efficiency by a factor of 3.4.
tl;dr - Given the enormous expenditure of ammunition required to bring down one aircraft (16,000 88mm shells) German anti-aircraft guns were probably a relatively ineffective use of scarce resources.
https://www.quora.com/How-effective-was ... nse-in-WW2
It appears that one of the problems with German heavy AA was that their shells did not have a proximity fuze. They were only time fuzed, set to explode after travelling to a certain pre-determined altitude. But often the altitude of the bombers was mis-calculated and thus the shells went through the formation or even the body of certain planes without exploding. Obviously, if equipped with a proximity fuze in these cases they would have exploded as well doing much more damage. The interesting thing is that although there were several proximity designs developed in Germany (including acoustic and electrostatic ones), none of these saw large scale service for some reason. Some sources claim these experiments were eventually unsuccessful, others that there was just no interest/willingness to further develop and mass produce these.
It looks like the standard German Flak fuze was the Zt.Z. S/30 mechanical time fuze with absolutely no impact action. Which means on some occasions the shell simply went through the wing or fuselage of a heavy bomber without exploding, since the the time fuze was set to explode at a higher altitude. So even a combined timed/impact fuze would have increased the effectiveness of German AA for which the technology was there from the beginning. Such double action fuzes were in fact developed by the Germans in the end but their large scale use was only to be conducted from April 1945, too little, too late.
On the other hand, the Western Allies did develop an effective proximity fuze and used it with great effect and they guarded it as top secret. Which means the heavy AA guns of these nations should probably be more effective in the game from about 1943. However, these were introduced later in the war when they had air superiority anyway, so their effect was somewhat limited. In the mod the Western Allies have relatively few heavy AA guns as they mainly rely on their fighters and adding more heavy AA guns would only limit the space available for their tanks and infantry to move.
So basically these are the reasons why the German heavy AA was nerfed from the vanilla game, and had their rate of fire drastically reduced from 10 to 7. But this change was originally done by deducter and not me. I just took it over as I did many other unit stat changes from his unit revisions mod.
I know that a lot of players are surprised or even mad to see that the effectiveness of the 8.8cm Flak is much reduced in the mod compared to the vanilla game, but the historical research shows that these were not very effective in actually shooting down planes. Their main use was that of being morale boosters, creating a "show" for the civilian population that the government does everything to stop the bombers. But heavy Flak also affected the crews of the bombers and on many occasions managed to disperse the bomber formation making it easier for the fighters to attack individual aircraft.
It think this is all well simulated in the mod now as AA can only do like 1-2 damage to an enemy air unit which may seem marginal, but yet it can just be enough for the fighters to finish it off.
As for the game engine limitation, obviously OOB does a better job in having two different air unit classes, small and large with small and large AA guns being effective against these, respectively. In reality a 2cm AA gun would be unable to hit a high flying B-17 because it was flying well over its maximum range.
On the other hand, smaller caliber, high rate of fire AA guns were fairly lethal against low flying aircraft. Thus it is good that tac bombers have low air defense making them vulnerable to these. The only problem here is that they are also vulnerable to heavy AA, which should not be the case.
This is indeed a problem in PzC1 (and sadly, as far as I know in PzC2 as well) and there is no good solution for that. There is only the work-around solution to give higher rate of fire and lower air attack stat to smaller caliber AA guns and the opposite to the heavies. To some extent it makes heavy AA units more effective against air units with higher ground defense and that's why high flying bombers and fighers have a higher air defense in the mod.
Perhaps it coud be improved by giving even lower rate of fire to the heavy AAs while further increasing their air attack. But the historical evidence shows that these heavy AA guns were not very effective anyway so it is up to further debate (and testing).
We had a bet with my friend ( 5 beers) who wins the scenario ( Battle of Moscow 42) and the result was decided in the very last turn... when my 5 str Partisan refused to retreat against a Tiger tank attack with only 2 entrachment 
