For me it was a red belly piranha and a banded leporinus, both have fast growth rates either way. I fed tetra min flakes and hikari carnivore food sticks.Interesting topic. On paper live food wouldn't offer anything more than quality pellets.
In a scenario where food is old or lacks any particular micro nutrient would hold back fish's growth and feasting on tank mate might solve that.
Just curious, what were you feeding to fish that grew significantly after eating a neighbor?