Unfortunately the "pound for pund" efficiency is in most cases academical. The 1600lbs less weight of the AP bomb don't make the Stuka more survivable or let you get that much quicker to your target, especially when compared to planes like the F4U-D or P-47N and so on.
So if you are chosing the Stuka at all, it only makes sense to take the bigger bomb with about 20% more damage and a much greater damage radius.
From a purely MA gameplay point of view, there is not much incentive to take the AP bombs, respectively to choose the light/medium bombers that carry them over the usual common bombtrucks