I like my vierling on PDH!
I was one of the main lobbyists in getting the flak damage reduced. Perhaps the radius or damage can be increased a little, but due to time constraints and such I feel that the new damage system for the 20mm and 40mm flak guns are much better than their 2.15 counterparts.
I honestly don't think that with the BF2 engine the 20mm can be done any true justice. It is either going to be too powerful or too weak. Like I have always used as my argument before, the damage they did in 2.15 was so great that if that is how they were IRL then they would have been wonderweapons! I am sure the main killer from flak guns is shrapnel and not concussive force which IMO cannot be illustrated in the game very well. IRL, a variety of factors would keep shrapnel in check (dirt mounds, elevation of the target and the trajectory of the shrapnel, trees, etc). However, the splash damage in BF2 is not so sophisticated to allow these kinds of things from reducing the general power of shrapnel-based projectiles.
Since these weapons have a high rate of fire, you can clearly see the difference in gameplay reasoning as to why they need to be nerfed a bit in comparison to tank HE, grenades, and other such ordinance.
Of course, this is my opinion and how I see it.