Yes, you can give the +mana trait to begin as a custom sovereign, but you gotta balance the non-customs too, no?
I've played with Magnar and Ceresa. Don't get me wrong, Dirge of Ceresa and Horrific Wail are fantastic for clearing out troops. Mages aren't underpowered. They just level more slowly because in early game, you only get mana so quickly. Haste/slow are also very nice lower level spells. I like the spells mostly, except for burning hands maybe which always seems pretty useless.
Meditation is a must. But it's not always possible to get high essence cities. Again, warriors would use that essence for something productive. Mages have to convert it into mana just so they can level up. Given the option, I'd RATHER have enchanted hammers in most cases. But no, I have to be able to get a shot off in battle.
There's already some compensation. Path of Mage mages get 25% off tactical spell cost. It's just not enough, I think. And I don't think that needs to be increased. I just think that the *first* mage needs a bit of a boost. +1 mana/turn on path of mage would be a large boost in early game, but less significant later when you have a few shrines already.
Note too, I'm saying that it's hard to level mages and when I play mages I always go for Potential and Knowledge. Because I want to hit those spell levels as quickly as possible. It's STILL slower than a straight out fighter, I think.