I'm wondering.
Does it only increase when it levels up, or when creeps up the heat meter?
Like.... would a level 5 tempered calibur with half of its heat meter filled do more damage than a newly level 5 tempered calibur with only 1/4 of its heat filled?
Not that I've tested this, but I'm pretty sure its per level. That would be too troublesome to constantly update a weapon per heat increment.