Well, in these situations, I like to go back and examine the wording in the rules... (if only so that I can remember what I was thinking at the time. )

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PIERCING [X]

Piercing weapons are more capable of pene-trating a target's defenses than normal weapons. The result of each impact die rolled in an attack by such a weapon is increased by an amount from +1 to +3; the higher this value, the more effective the weapon.

INCREASED IMPACT

The damage caused by a weapon with the increased impact trait is affected by the degree of success of the impact roll. The number of points of impact caused by such a weapon is equal to the amount by which each impact die exceeds the target's shield rating.

Against a target without shields (either by design or as the result of damage), the impact roll should still be made—the number of points of impact is equal to the result of each impact die.
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So, Piercing increases the result of the impact die, and Increased Impact compares the result of the impact die to the shield rating, so I would say your assumption would be correct.