A good combo for hunting undead might then be Silver tipped, Yew shafted, Owl feathered arrows with a Fire IV enhancement which would do 1d8 piercing + 1d8 Bludgeoning vs. Undead/Shapechangers +1d8 Fire damage.
In your above arrow example, you'd get +1d6 piercing on top of the 1d8 from the arrow, unless you were fighting constructs, against which you would receive +2d6 piercing on top of the 1d8. The question here was whether you would in fact receive 3d6 vs constructs, which turns out to not be the case. Also, I added a note at the bottom of [lore]Item Enhancements[/lore] which deals with this case.Actually, it would be +2d8 Bludgeoning vs Undead/Shapechangers. And the 1d8 fire would have overwritten the 1d8 Bludgeoning the arrow normally gets when not facing Undead/Shapechangers. However, this means that adding an elemental enchantment, even though it overwrites the 1d8 bonus damage that came with the crafted arrow, is actually better vs undead than an arrow with an alchemical silver enchant on it. This is because the "vs" bonus damage and the general bonus damage are no longer of the same type and can now stack.