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It's been a long time since I've heard about this in specific (IIRC it was from one of the videos from ullillillia) so I might be wrong, but I think that, at level 99 (max), a spell will deal triple damage when compared to level 1. I don't know if the growth in power is linear, but I believe it is.
Regular buffs and debuffs don't get any power per se when leveling up, so you shouldn't grind them for more than 4~5 levels (assuming you have some weapon mastery in staves and is equipped with one. If not, go for lv 9), which is for the AoE and spell range. You'll just be wasting more SP from leveling them more (the SP cost still increases with level). Similarly, don't bother leveling up skills like "kneel before me" from Rozalin, the buffs from Taro/Hanako, etc