Breach enchantment should partially ignore shields too

Breach reduces the damage reduction of armor by 15% points per level. This means that diamond armor reduces damage by 80%, but against a breach 4 mace it only reduces damage by 20%.

So it would make sense that a breach 4 mace would reduce the damage reduction of a shield from 100% down to 40%. The remaining damage would then be further reduced by armor (also whilst still affected by breach), and then by protection enchantments.

It would provide more uses to breach in PvP, since density seems to outperform breach after only falling 7 blocks. It could also become useful in PvE if mobs ever gain the ability to use shields (which I think they should).