What happens if, after suffering damage, the player summons Reno Jackson, triggering his battlecry? Is 15 the new permanent maximum, or is 8 still considered "full health" for Ragnaros?
From the wiki you linked:
Alexstrasza can be used upon the Ragnaros hero to alter its current and maximum Health to 15. Following this, if the Ragnaros hero takes damage it will be possible to heal it back up to 15
This was apparently confirmed by Blizzard employee Zeriyah on her now empty Twitter.
Ben Brode has also confirmed via twitter that the new maximum HP is 15.