How do you plan to determine productivity? I think it would be a good idea to give a bonus if an employee is doing a really good job, hustling, etc., but I don't think I would set some criteria that based pay on productivity on a week by week basis.
Give the workers that hustle bigger raises and/or give them a bonus check during a team meeting so that the other crew members know that they might get a bonus if they hustle, but I don't think a weekly pay supplement would be good.
I just wouldn't recommend setting up a scenario where employees start to depend on a production bonus and then feel let down/cheated when they don't get it.