I don't think it's fair for you to say that your build works with Coil but wouldn't with the chest in question... 5% less phys damage taken conversion isn't going to break anything, unless you just have no life, but I don't see you utilizing Gluttony of Elements or otherwise achieving full elemental immunity, so I assume you have a little bit... not to mention that it opens up other slots with resistances because you're essentially gaining 175% elemental resists just changing Coil to this chest. You could also gain an additional 14% phys damage taken conversion with an optimized Watcher's Eye, which would overcompensate for the 5% you're losing here.