I would suggest that you perform a very simple, controlled and repeatable experiment.
1- Take a graduated jar which can hold 1 liter of water (I'll be working in Metric as it is far easier to convert everything into watts).
2- Measure the initial temperature in Celsius
3- Put the water inside your oven, set it up for a nice round number, 100 seconds. Do this at nominal voltage, 120 volt. I assume that you will be measuring electrical power.
4- Remove the water from the oven, stir it a little to homogenize the temperature, and measure the new temperature.
5- Calculating the temperature delta between 4 and 2,
6- and knowing that a liter of water can hold 1000 calories for each degree Celsius of temp rise, you can calculate how much energy the oven actually transferred to the water.
Repeat 1 thru 6 but now at 95 volts.
If you took input watt readings for both conditions, calculate its ratio. And then the calorie ratio for both conditions.
You can then determine a relative figure of merit between both ratios.