Well, she seems to be able to defeat Yami, Espada 10, pretty easily. Granted, he doesn't release his sword but then again Yorouchi doesn't really break a sweat either.

Can Nel do a cat to awesome fighter type transformation? Well, we know one thing which is during the fight with Don Pannini her mask suddenly seemed to come to life, but nothing happened of it. Also, one of the goofballs said something might happen to Nel if he isn't there to keep watch over her or something.