Doh... I keep forgetting that the Diadem is useful for other casting classes tooOf course it has the other casting stats on it.
The ring of elemental essence is basically potency 90 for the elemental spell power.. only thing (that you care about) it is missing is force and nullification, which you have slotted elsewhere -- so, yes it is not a bad option. Swapping the ring can make sense, as you really only need the potency if you are going to dot up a mob.. you have Corrosion, Nullification, Combustion, and Force slotted elsewhere -- so using it as a swap can work.