That would depend on the design of the driver. Some drivers may reduce the light output if the input voltage is reduced that much.
Other drivers may draw more input current (same power out with reduced voltage in means more current in).
The efficiency of the driver may be reduced at the lower voltage/higher current levels, and you will have a little more voltage drop in the branch circuit wires.
You can try Gar's suggestion, but if you are operating outside the drivers specifications, there is no guarantee 10 other lights will operate exactly the same as one you test. And there is no guarantee everything will remain the same for different ambient temperatures, and as other factors change.