I'm pretty sure that's the increase in enemy penetration, not blocks.
One would think you need to modify the "BlockDamage" property. The catch is that "perkPenetrator" doesn't have this property, so in theory it shouldn't increase block damage at all. (Other perks do, though.)
So, I think what you want is "BlockPenetrationFactor". From the comments in buffs.xml: "This many block hit points reduce EntityPenetrationCount by 1. 251 is just enough to penetrate 1 wood block or door." This property is set on the ammo itself, and I don't see any buff that affects it.
From the numbers, I'm guessing that's where TFP get the "1000" value? It's close to (3 + 1) x 251, meaning: AP ammo penetrates 1 enemy, the perk adds another 3 enemies, and each enemy is equivalent to 251 block hit points.
I haven't tried this myself, but that would be my guess. Hope it helps.