I don't have any hard data, but my observations are it has slowed down the horde, if anything. Personally, I think the bunkers were to soft anyway. At least now it takes 850# of ordinance to take them out. Bombers will still bomb, Jabo's will still dive bomb, it just takes a little more effort now. 
How about a compromise? When I attack ammo bunkers, I use rockets. Three rockets for each ammo bunker before the hardening was just about a sure kill.. How about setting it to 4 rocket hardness that have to be well placed so there is no room for error. Five rockets would give you a bit of wiggle room..
Four rockets would bring back in the F4u1D as an example carrying 8 rockets for ammo bunker attacks, but a five-rocket attach using the P38 or P47 would be a more reliable rocket attack platform. Tank wise that would take 3 HE instead of four HE now.
A compromise is even more important when a map requires long air or tank routes to the next air base.
Just a thought.