Or even just a straight +20% damage vs. burning targets
Would make whurrgarbl too important an item for the wizard.
My point is that Paladins can get access to more mana than Wizards, which means unless you're using PISORF over BURNDAYRAZ, the Paladin can cast as many or more spells than the Wizard.
Even under the most extreme scenarios, the Paladin will fall slightly short of the wizard in terms of spellcasting power. In any realistic scenario, the wizard will virtually always have one extra fireball over an equivalent paladin. He's still the better caster here.
A Paladin would need 18 mana to be about an equal spellcaster to a wizard with 15 mana, and he'd need 24 mana to be about equal to a wizard with 20. If we consider only
the boost mana boon of Jehora Jeheyu, then the Paladin is roughly on par with a wizard so long as he's one boon level ahead. However, this advantage will only exist for a very short time, since the wizard will only need 10-20 more piety to catch up. Furthermore, if there are any
other max mana effects in play then the paladin will still be definitively behind the wizard even when he's ahead in terms of the boon. On top of this, even with an equivalent mana threshold the Paladin still doesn't get that nice synergy with BLUDTUPOWA or the crystal ball.
With Mystera and Earthmother, the inability to convert is a huge
blow to the Paladin. With those deities, monotheism is a curse, not a benefit. I'd give up the 20% discount and punishment immunity in a heartbeat to be able to convert in and out of those deities.
Again, the Paladin is still a top-tier class that puts the wizard to shame. However, the wizard is still the unambiguously better spellcaster between the two.