Clever wording on their part: 2x performance per watt over M2. Took me a minute, had to reason through this is their 2nd generation 3nm chip, so it wasn't from a die shrink, then go spelunking.

This claim can only be evaluated in the context of a specific operating point. I can 6x the performance per watt of the CPU in this machine I am using by running everything on the efficiency cores and clocking them down to 1100MHz. But performance per watt is not the only metric of interest.