Yeah, she'll do 9999 easily at fairly high levels if her limit gauge is nearly full, and her limit itself will do ridiculous amounts thanks to the gauge being full when you activate the limit. I personally never use Premium Heart any more, it's sucky how little damage it does while she's on low limit. Of course, you could leave her on full limit all the time and make use of command materias to enable 4x cut, Slash-All or whatever so she can still do physical attacks while on full limit.
All the characters require certain conditions for their weapons to do Max damage BTW.
Cloud (Ultima Weapon): Depends on his HP.
Barret (Missing Score): More AP on his materia the better it gets.
Aeris (Princess Guard): Double attack power when 1 character is KO. Triple attack power if 2 characters are KO.
Red (Limited Moon): Depends on his MP.
Yuffie (Conformer): Weapon has no powerup, it's rock-hard by default
It's best used for Morphing enemies.
Vincent (Death Penalty): Depends how many enemies he has killed. More is better.
Cait Sith (HP Shout): Same as Cloud.
Cid (Venus Gospel): Same as Red.
edit: this only applies to their
ultimate weapons, just to clarify for anyone who didn't realise. I've added the weapon names now as well to the above list.
