I must be missing something: I was under the impression that the category of the dragon detemines the amount of HP the dragon has and the amount of damage the dragon deals; and that category 9 was the highest (excluding category 10, heroics).
However, I encounter more and more dragons in the arenas that, despite having the same or worse category (and less than or equal the amount of stargs) - yet have more HP. For example:
Here I have the Blade dragon (category 9), against Dragem (category 8). Both are level 26 and both have 3 gold stars. Neither is boosted (when boosted, the HP bar is highlighted yellow). Yet the Dragem has about 30% more HP than Blade.