1. When she was a slave, she killed 50 of the empire's best men with ease.
2. When she first went to Esper form, she flew at incredible speeds (I forgot how fast).
3. AFAIK, she was needed to Humbaba who was powerful enough to give the party problems.
That's it. Though she naturally learns Cure, Fire, Poisona, Drain, Raise, Fira, Teleport, Cura, Dispel, Firaga, Arise, Holy, Break, Graviga, Meltdown, and Ultima. As an Esper, she's at least twice more powerful than her human form.