Clearly there is significant opportunity in delivery. If you are already open for business and paying the rent etc, the sales are incremental and can add to profit nicely.
Not sure what your “four pizzas per driver” refers to. As Pizzamancer points out, deliveries per hour is what you need to focus on.
Drivers can not be contractors. You are taking a major risk if you try to go that route. Talk to your CPA and attorney about it. You write the work schedule, you set the compensation, you provide things like hot bags… they are employees and you pay work comp, unemployment etc.
Insurance is not cheap. You CAN NOT rely on the drivers to have their own coverage. You need to buy hired and non owned coverage to protect YOU.
Delivery is a competitive market, you will need to invest in marketing to drive sales.
If you can add $500 per night in sales it is probably very worth doing when added to an existing sit-down operation. If you are thinking that adding $100 per night will be break even, you are mistaken.