As a Paladin, I approve of the use of the Conjuration Sphere using my class level instead of my actual caster level. It means I don't have to spend a talent on Boon Companion to have it not keep up anyway. I am the Angel Summoner AND the BMX Bandit in one!
If I we're to make this a feat, I would instead have it grant a +1 to your caster level with a Sphere of your choice that doesn't stack with class abilities that grant additional caster levels to a Sphere, such as the Wizard/Sorcerer/Incanter/Theurge abilities that grant +1 to +6 CL to a Sphere.
Full caster level is too powerful for a low caster is my point, the lowest of which are Paladins and Rangers at max CL 8. You could grant even these two a full caster level companion, which can then take a tradition that includes this boon, and gain a full caster level for whatever they want. Pocket healer, bard-like buffer, witch-like debuffer, etc.
My Mageknight would enjoy having the Warp Sphere be at full caster level. Its not terribly powerful, but being able to reposition an enemy 3 times a round while dealing 1d6/2 levels to that enemy each time is really powerful. And you can do that in an AoE spread, and they move anywhere on the battlefield you want. I would go out of my way to pick up any +1 level metamagic feat and Quicken Spell to make this happen.
What about a boon or feat that simply increased your caster level in one sphere to the next rank up? Low-caster -> medium caster, medium caster -> high-caster. It's simple, gives a wide benefit, and prevents a single talent from suddenly becoming potentially much, MUCH stronger than everything else. Plus, there's already a precedent for it in how Elementalists and Shifters work (plus I may be forgetting a class or two).