Actual current flow and actual wattage are based on actual, measured, voltage on the system. Actual voltage in system is almost never exactly what the batteries are "rated" at. Depending on the state of charge of the batteries, type of batteries, and the total load on the system, the actual voltage can be quite different than the rated voltage, high or low.
To try and answer the original question, no the rerating formulas for bulbs are not linear, and it would be less than 20% more light output.