It turns out that, for effort put into someone, you can get about the same percentage improvement. But 10% more of a star results in a superstar. 10% more of crap is still not good.
However take something that sounds like a step in an assembly line. Data entry. One of the examples in First, Break All The Rules was a data entry group that took the woman who had the highest data entry speed (and also significantly lower error rates) and worked with her to see if they could do anything to improve her work, or to replicate her abilities.
In the end she sped up considerably to what they would have thought previously to be an utterly impossible speed, with even lower error rates, and several other people on her unit wound up with data entry rates that exceeded her initial already stellar levels.
The fact that a job is simple and boring does not mean that it can't be improved. And the people who both have the most potential to be improved, or to improve it are generally the people who already are best at it.