Is it too easy to cast powerful spells with RPM?

[Aldric]

Isn't a caster skill limited by 12+magery anyway ? How would someone get in the high 20s range ? A grimoire for +4 (but doesn't that double the required time ?) and a +2 for a specific ritual with a perk.
That's 10-11 energy per attempt on average, and a basic Fireball costs 18.