Multiplicative scaling of 10% will always be a 10% increase, no matter what other buffs you have. It's just simply theoretically impossible to go from 1600 to 1900 with piercing debuff alone. And if I'm not totally off-base, it's closer to a 8% damage increase because of your dots not benefitting from it.

And as a random bonus nitpick, 10% reduction in resistances translates to ~11% increase in damage: 1/0.9=1.111.