As far as I can tell, gaining 3 caster levels for evocation spells, which you get from the 3 piece set is helpful only when casting 2 spells- polar ray and chain lightning.
However, if one is to get all 5 pieces, you also gain 3 caster levels when casting conjuration spells. After looking though the spells, I cannot find a single spell tat would benefit from this increased caster level either by doing more damage, lasting longer, or really in any way whatsoever.
I have 3 alternative suggestions: Make the 5 piece set grant you 3 caster levels for all spell schools.
Think that is too overpowered? How about adding some damage based conjuration spells that increase in potency with caster levels.
Too time consuming? How about changing it to give the feat "Augment summoning" instead.
Otherwise there really is no reason to get all 5 pieces that I can see. My caster would actually go to an even strength from the +3 profane con bonus and gain 40 hitpoints, but I would have to sacrifice my shroud 45 HP item to use the slots, causing me a net loss.