I think I would max the costume in almost every case, just for the stat bonus. Maybe not if the original hero were at 1^1 AND I didn’t plan to use the costume, but otherwise, costumes are relatively cheap to level and take almost no rare mats. And the full bonus is the same as having like 5-6 talent levels. Not much of a downside.
Definitely don’t overlook the trials aspect. I leveled Skittles’ costume only because I needed some harder hitting clerics. And because there aren’t many good sorcerers on my roster, I have a Skittles +c18, emblemed along the attack path for clerics. Her attack stat is nothing to sneeze at.
In your case, for Boril, I would probably max the costume.