I've noticed that different dragons get different amount of boost in life and damage from being empowered. Here are examples I have so far:
Destiny, common, 3 stars, 20% extra
Pure, epic: 1 star, 14% extra
Boulder, legend: 2 stars, 76% extra
It would be great to see more examples, so we can figure out the rules.

Also, I noticed that two different epic, cat 5 dragons had different recall times. I wonder what that depends on! Given how long there recall times can be, I it would be nice to see what affects it For example, does it depend on the number of orbs one is getting?