It's contextual but that doesn't mean that "best" doesn't exist.

At the beginning it's unlikely to be tried on humans, but when it comes to farm animals I would expect that people do experiment with it. Maybe a pig with yeast mitrochondria does better than a regular pig because the yeast mitrochondria had more time to be highly optimized by evolution. It might be that you need to do a few additional changes to have the pig deal with the yeast mitochondria, but people will experiment.

If you get an enzyme that's twice as efficient in catalyzing some reaction, you have to down regulate it's expression.