I don't know.

I did not hear any great demo of how Pa800 (or Pa900 for that matter) deal with the Vocal Harmony in real life.
But I did hear Harmony M.

It's true the Harmony M only have 2 voices, only 10 presets, but the QUALITY is great.
Here's one or two links. Plus those on TC Helicon website.

http://www.youtube.com/watch?v=WXkV945OngQ
http://www.youtube.com/watch?v=4hTZSkZ2kU8

Can anyone post something of Korg Pa800 or 900? Some real songs like these.
I honestly would like to be prove wrong that Harmony M is better!