Yeah, unlimited uncapping can be really strong. An 11d6 fireball at 11th level is a 10% increase, but a 20d6 Shocking Grasp at 20th level is a 300% increase. I would increase damage caps by two times level instead of one times level, but that's just me.

How does Mastery of the Element work with prepared casters? If I prepare a Quickened Fireball in a 6th-level slot, do I still have to use a 7th-level slot when I Energy Admix it?