Short question:
Is there or will there be an equivalent of Mio PAI in our watches?
I think it's just a matter of proper calculation algorithms, no additional sensors needed.
I think they are called PAI Health now - https://www.paihealth.com.
If that's what you mean, yes they integrate with Garmin - on the server side rather than device. As soon as you sync your watch you can view updated PAI stats either on PAI's mobile app or web site.
PAI Health is a seperate company, which owns the intellectual property and has their own business model. Garmin now owns the consumer facing portion of Firstbeat, so I'd guess that this is the similar feature that's most likely to appear in new devices - www.firstbeatanalytics.com/.../