From how I'm reading the focus effect, it will save 20% on all detrimental spells. It says no complete heals, but Detrimental Allowed.
If that's true, ANY class that casts detrimental spells would gain HUGE benefit from this. Normal Mana preservation is 10%, Elemental quest gear Mana pres is 15% mana savings.
IF this would work for all nukes/debuffs/crowd control/etc. Then this would be amazing for any class.