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You could still measure the efficiency of the driver over various input voltages, output voltages, currents, temperatures. You’d get a huge number of measurements for sure, but you could condense them to some key numbers/indicators like “efficient for low brightness”, “not efficient at all” or something else people might think about.
I think it’s a great idea to measure those numbers, but probably a lot more work than most of us would be willing to do, and of limited usefulness.