from my point of view, giving a "temporary" buff, is a big nerf.. but do as you wish.
on Magistermodmod, Fair Wind is temporary (dispels in about 5-10 turns). it's a pain in the ass: you cast it once when ship is born, and then, forget. the micro-management of having the ships come back to re-equip is not worth the gain.
(the only perma buff I see are : loyalty (limited interest) , valor (limited to 100xp), courage, enchanted blade, poison and fire-arrow : any of those are important, and making them temporary would mean that you would need an adept/mage in close distance : less than 3 turns : that's a pain in the ass.)
even "bless" is a pain as it's until combat only...)
I don't know how you'll code it, but unless you want to reduce the power of arcane units, I expect that the spells will need to at least stay at the level they are now but give increase of power if mage specialize.(otherwise a versatile mage will be weaker than in actual game, which marginalise arcane units... and the increased power due to specialization is not really an increase... because it comes with the cost of versatility !!)
ex : enchant blade, (lvl 2?) : normal : +20%, applied melee / mid : +25%: melee/mounted / high : +30%melee, mounted, disciples
ex: fire arrow : normal: +1str / mid : +1str +10%CA / high : +1str +10%CA applies to range & siege.
..etc
(I don't know how you'll balance, but I hope that a mage specialized in Air will have an Air II much stronger than RoF : the balance between mage and Cultist (5str, access to command promotions, starts with RoF from scratch and not at level 4, and RoF is stronger than Air II) is that mage are supposed to be versatile.... however if you need to specialize (ie, sacrifice the versatility), you obtain an expensive unit (needs lvl 4 + gold for upgrade), weaker than priest : 4 str, no access to priest equipement for str... conclusion : a mage specialised in 1 school should be MUCH stronger than a priest's spell.... and a mage with 2 schools should have each lvl 2 spell at least at priest str value.