MechaPilot
Explorer
Maybe it doubles the duration of divination spells? Maybe it gives +1d6 damage to rays and cones? Maybe it grants +1d6 to CHA checks because of how awesome it makes you look when you cast?
I looked at the math around spells and saving throws and assuming all parties in the game universe are treated similarly spells are hard to resist and get harder if the opposition levels as quickly as your character does. This gets worse if the caster can have additional features that increase the DC.
I feel like you are looking at +X foci in a vacuum because the balance to +X foci is +X save boosting items, just as +X armor is the balance to +X weapons.
Now, I'm not big on the whole +X thing. I find a static bonus that fades into the background to be rather boring, so I prefer more active abilities. However, adding +X items are not game breaking as long as you arm both sides of the conflict with them.