Does higher mileage lower the residual value?

Yes. Residual values are set for specific mileage allowances. A 10,000-mile/year lease will have a higher residual than a 15,000-mile/year lease on the same vehicle, because the car is expected to have fewer miles (and therefore more value) at lease end. Choosing a higher mileage allowance increases your monthly payment because the lower residual means you are financing more depreciation.