Then one possible way of implementing that is with the human form limitation.
For example:
-2 Inhuman strength
+1 Human form, covering:
-2 Supernatural strength
So, if you are just 'normally' angry you are only inhumanly strong, but if you get really angry, you become stronger, but it's obvious that you are no longer normal/human.
Another thing to loot at is the beast change power, that allows you to represent getting better at splitting skulls as you get angry, and forgetting all about your flower arranging skills.