Because I don't write paranormal romance or even paranormal novels and don't read widely in those genres, I'm not knowledgable enough to address the question of a Buffy Factor in YA literature. I do think Buffy, herself, was a high point in television as far as powerful younger women are concerned. I don't see how anyone who was a follower/fan of that show can read the Twilight series and not be uncomfortable with Bella's passivity and child-like dependence on a man. I sometimes thought of Buffy while reading those books.
Hmmm. So while I don't know if there are any BTVS parallels in YA literature right now, I guess I'd have to say the show had an impact on me as a reader. I want some strength in the female protagonists I'm reading about.