Pai is top tier. I think Akira is the best o the VF characters, but I'd put Pai above Sarah. Not by much, but enough. As a former Helena main, I looked into Pai. She's harder to play than Helena and will require far more work to be good with her. You have to read your opponent very well. She's like the closest thing to a DOA character out of the VF characters. Once you get going with her though... yeah, you're done. And I feel really, really bad for any character with an 11-frame jab or slower.
As of now, Rig and Mila are my mains. Pai will be up there as well, but I need to play against people with her more. Training mode isn't enough for me to really figure out if I like the way she flows. She can get really dumb if the opponent starts to block though. Wow... seriously, just let her hit you, things will end better that way.