Got a fast and remarkably reply from someone at Helicon itself :

"...Dear John,

Thank you for contacting TC Helicon Support.

I'll be honest with you here - since we've never supported the PA800 or the Helicon functionality inside it, it's a bit hard to compare directly. But from what I've been able to dig up, it features functionality that's somewhere in between a Harmony-M and the higher-end VoiceLive 2.
This boils down to: you wouldn't be getting the full functionality of the PA800 with a Harmony-M...."

So if this is true it would be a waste of money, or so it seems...

regards,
John