I was under the impression that your overall damage was decreased, but you applied more damage to the hull than normal. In the case of the OP, I'm guessing 19 additional damage (possibly affected by skills and weapon power?) was being added to hull damage, but his overall weapon strength was being decreased by 15%.
In any case, one of the best rules of thumb about shield penetration is that if the penetration ratio is small than the shield:hull ratio, you're wasting your time. For example, say you have an ability that gives you 10% shield penetration (and assume there is no bleedthrough for simplicity), your target has 100 shields and 500 hull, and you deal 10 damage per shot. Each shot would deal 9 damage to the shields, 1 damage to the hull. Obviously, since you're dealing more damage to his shields, and he has more hull hp, his shields are going to fall before you kill him with hull damage anyway.
Directed Energy Modulation seems best suited to finishing a target off that's at low hull but has managed to get his shields back up, not for general use.