I am suggesting this for the following reasons:
- Gym Leaders are currently Equal Trainers, but the Gyms have ordinary Equal Trainers as well. Furthermore, Gyms also have Epic and Legendary Trainers, which makes the Gym Leader's status as the Gym Leader not the most obvious thing. While the designs of all the current Gyms kinda put the Gym Leader at the end right now, if a future Gym ends up not doing a very good job at highlighting the Gym Leader, a Boss type named "Gym Leader" for the Gym Leader will make it clear to the player who the Gym Leader is. That's the main reason for this suggestion.
- Gym Leaders are Equal Trainers right now because having them too high level causes them to be too powerful. While Speed was always the main issue, the other stats were problematic too. A +15 level increase hopefully won't overwhelm the player too much.
- The Legendary Trainer is often much more challenging for the player than the Gym Leader simply due to the +40 level increase. It can beat down the player with sheer power. While I doubt a +15 level increase on the Gym Leader will change that, it will at least help the Gym Leader pose more of a threat.