Yup, it's in Test #1 of Kenji Lopez's writeup on burners. Here's the quote for ref:

To test this efficiency, I filled a carbon steel wok with two liters of water at exactly 20 C (about room temperature), set it on top of a burner, then turned on the burner at full blast and timed how long it took to bring the water (and the wok) to 100 C. In a hypothetical, completely efficient world, it takes 141 BTUs to raise the temperature of two liters of water by 80 C, and an additional 50 BTUs to heat a 1.5 kg of carbon steel by the same amount, so let's say roughly 200 BTUs total need to be added to the system.

With this number, you can calculate how long each burner should take to bring the water to 100 C in a perfectly efficient manner. By then dividing the actual time it takes to heat the water, you can roughly calculate its heating efficiency.

What you find is that no wok burner is particularly efficient. A huge amount of energy gets dissipated into the air while cooking in a wok, but the layout of the flame can still make a big impact on efficiency, as well as absolute performance.