If South loses his desire and excitement to fight he can't become stronger anymore, and he most likely goes back to how strong he was in his base form. This is obvious Empowerment. Mikey's dark impulses peaked in chapter 262, since during his fight with Takemichi he started becoming more and more sane and less berserk, starting to gain more control over himself, resulting in his emotions that trigger his dark impulses to become weaker. Because of this, it can't be Empowerment or Rage Power, and it can't be Statistics Amplification either since it's not like he activated something and became stronger on purpose. It would be battle type
Accelerated Development. It pretty much raises the person's power level slowly during a fight, which can allow the person to adapt to someone stronger, and beat them. So in conclusion, when Mikey gets taken over by his dark impulses he doesn't suddenly just become stronger, but it allows him to become stronger if he fights someone.