Patch Available - KMST 1.2.456

I want to see what Additional Potential does to the stats of lv 140 items when they become lv 180.

I don't think that'll happen. In very basic terms, the additional levels imposed on some equips was meant to apply only to equips that weren't level 140 in the first place while < level 100 equips could go up to level 130 and have a different set of options compared to a similar equip that didn't have an additional level requirement.