Jack Stack wrote a book called "The Great Game of Business" about his incentive system. It is one of thousands of business books, but his take on piece work is from personal experience. I recommend it, among others, before starting down that road.
Just a few of the problems that will have to be answered, preferably before starting:
Who decides who runs the gravy jobs, and who gets the low pay jobs?
Nobody wants to change the inserts 10 parts before shift end and let next shift get the gravy.
When shop buys better tooling or other job improvements, who recalculates the job time, even though worker is doing exact same loading and qc checks?
You will have to pay minimum wage if employee doesn't make enough parts to make minimum.
As soon as the system is announced, both management and workers will be working their hardest to game the system. To the OP, if, in your example, the operator was able to change things so they could make $40/hour and still turn out perfectly acceptable pieces, what would you do? If you would find that acceptable because you are also making more money, how will you handle the internal conflict with other employees who are making less than half as much?