Per the lovely thing that is Elantthipedia;

"Focus Power simulates the effects of using more mana than you actually did (this may increase spell damage). This bonus increases as you invest more mana in the spell, until you approach the mana cap. At that point the spell plus the bonus is considered as powerful as it's going to get, and the effective bonus decreases -- until it becomes exactly zero when you hit the cap."

"So in a nutshell, Focus Power does effectively nothing at the min prep and mana cap. There's a 'sweet spot' involved in getting the most out of this particular pathway." -- GM Wythor (11/15/07)

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"...I am inclined to think the focus of the [Warmage's] spellbook should be ways to make things explode, to help you make things explode, or to assist your victim in exploding." -Armifer