I like it with the spell mastery requirement. It would read something like this:
Studied Spontaneity [General]
Pre-reqs: Spell Mastery
Benefit: You can "prepare" one spell slot in a special manner. When you use this spell slot you can cast any spell for which you have mastery spontaneously. Only spells with a casting time of 1 standard action, 1 free action, 1 immediate action, or 1 swift action can be cast spontaneously but no matter the original casting time, the spell requires a full round action to cast [not to be confused with casting time of 1 round].
Special: A character may gain this feat multiple times: each time it allows an additional spell slot to be prepared as a spontaneous slot.
DC