Tips from Maxim Integrated on how to measure efficiency on complicated multiphase converters. Link here
Due to the nature of multiphase buck converters, perceived efficiency varies for a static operating condition depending on the load and output voltage measurement connections, as well as the symmetry of the PCB layout. An engineer evaluating multiphase buck converters should understand the nuance of efficiency measurement that this article explores, as well as PCB layout. How to fairly compare efficiency of multiphase buck converters on different evaluation boards needs to be addressed. This application note explores the underlying reasons and offers a method for measuring the most accurate efficiency in a multiphase buck converter.