Regardless of how good an installer might be, when you start talking about incentives for cutting time off of jobs you run the risk of having them cut corners.

Take the total of all money for all installations and let it add up over a quarter. At the end of the quarter, take a percentage (20%?) of that total and split it up between the installers. Not an incentive but a bonus. Different language - same bottom line. Any call backs are deducted from the quarterly total, thus less bonus paid out.