Did a quick test to confirm weapon favor bonus. Starting with one empty slot in the weapon grid and these starting values:

Chronos (wind) atk: 16234
Total weapon atk: 7976

Added electric lance atk: 611
Chronos atk: 16845 - 16234 = 611
Total weapon: 8587 - 7976 = 611

Replaced with water glaive atk: 819
Chronos atk: 17216 - 16234 = 982
Total weapon: 8795 - 7976 = 819

819 weapon atk * 1.2 weapon favor bonus = 982 atk

So that's implemented at nutaku. It's a boon to anyone who hasn't focused on an element for whichever reason.