It doesn't help mileage, all it provides is an increased risk of damage to your engine.
The goal of a good intake system is to provide the proper fuel to air ratio for output demand, with liquid fuel the system atomizes the liquid and provides the right balance of air to support combustion. It of course is much more complicated than this because you have to deal with flame propogation, surface area, quench ratios and of course fluid dynamics.
One challenge is to keep the fuel atomized because the smaller the mist particles, the greater their surface area and the greater the efficiency of combustion. GM developed the Vortec cylinder head design to optimize the fluid dynamics of the fuel/air mixture but it is only after extensive testing and optimum conditions that the Vortec works well at all.
There is NO WAY an upstream mounted diffuser can impart a worthwhile vortex that will sustain through the intake path, much less the intake valve cycles. If anything this device will impair flow and decrease atomization of the fuel/air mix resulting in less output for the same amount of fuel.
You can believe whomever you want, I'm an Engineer of large rotating machines and have a fair amount of experience with turbines and IC engine design
