I'm not a Jonin Medal user but i'm gonna give it a shot .. I guess that 15% bonus is for the Damage bonus, but i'm not sure if that's for the whole team or for your Main Character only .. In addition with that, damage in Nine Tails Invasion is not accurate and not always the same number .. I for example, my Main Character's skill delivers 240k damage at the 1st and 2nd try, regardless of the round, and when i did my 3rd try my Main Character's skill drops down to 80k even tho the whole elements is there, the barrier and the buffs, just like the 1st and 2nd try .. then at 4th try i goes up to 180k and so on and so forth ..

Anyways, lets wait for someone who knows well how this power-up thing works in Ninetails and gives you enlightenment with your query ..