Long time lurker, but thought I would try to lend a hand. Won't be able to help with the timing of the special editions.
I've been keeping track of this out of curiosity as well. Looking to time a purchase of a 2017 PP.
Here is what I've collected so far:
Links to Subaru announcements:
2015 Pricing
2016 Update Information
2016 Pricing
2017 Update Information
2017 Pricing Information
Hopefully this helps a little!