Yup, they don't stack. Else you would see mage wearing Q4 everything dragon / infernal. Bone chest is 29 less encumbrance than FP chest. So if you care about your bow damage, better forget about this chest-piece.

Now, we can check what's the exact damage difference between infernal + fp and infernal + fp with bone chest to check if it's worth losing 3 points resist in elemental and 2.5 in melee and the possibility to get some bloodcrafted FP chest.

[Linked Image from w3.the-kgb.com][Linked Image from w3.the-kgb.com]