The focuses on the t8 armor for pure casters seem to be of pretty limited use - unlike the melee and priest ones, which appear to be spot on!
Bonuses by piece and classes that can use them:
Boots - Frost (Wiz, sort of Mag)
Bracer - Poison (Necro only)
Gloves - Disease (none, since necros don't have a top-end disease dot)
Helm - Preservation (ALL YAY!)
Pants - Minion (Mag, Necro)
Sleeves - Fire (Wiz, Mag, Necro)
Other (jewelry) - Distance, Health, Chromatic
So it seems to me that it would be of great use to most silkies if the gloves had a Thunder focus, which would be of great application to 3 classes (I don't think mage has a magic based spell at this level). The bracer also seems to benefit only one class, and I personally think it would be great if that had Quickening instead, which all 4 classes would get great use from. The Poison focus could then be moved to the Necro-only drops from T7/T8 (I apologize, I'm not familiar with how broadly available that focus is otherwise). I do understand that poison is a critical focus for necros, so I wouldn't object to it staying either though!
Something to think about moving around or at least in terms of future armor set design! Thanks

EDIT: realized that mages don't get good use from a thunder focus