In my own campaign we originally treated them like all other spells, but our spellcasters were powerleveling much faster than other classes.
I'm really curious about this part. You've mentioned it before, and it's certainly not an effect I've noticed. Do I infer correctly that your spellcasters are/were levelling faster because they were rolling a lot more because they were casting a lot of spells?
So, every one of those xp came from a failed die roll. What kinds of hard moves were you making on those failures? ("Take away their stuff" is legit, which includes applying the partial-success consequence of forgetting the spell.)
More importantly, why were the other characters not doing more stuff? Players can roll Discern Realities and Spout Lore pretty much any time they want to, for instance.
And if you're keeping their lives filled with adventure, shouldn't there be a lot of Defy Danger rolls all around? What is the Fighter doing while the Wizard is casting his Magic Missile? If you have a passive player, then maybe you need to make a point more often of asking them what they're doing, or making them respond to an enemy threat.
I *have* seen characters race ahead in experience, but generally it has looked a lot like random luck, when someone at the table just keeps rolling like hell.