The best you could aim for is asking people whether they noticed a change in MPG after installing their supercharger. But the problem here is that we're talking about major performance mods that are likely to cause you to change the way you drive. how much are the results impacted by changes in driving habits caused by the availability of more power? And will the same mod cause you to change your driving habits the same way? Probably not.

The point is that there are so many factors involving location, and personal driving habits/patterns that you are not going to get any valid information about which supercharger has better fuel economy by asking people on the Internet unless there are extreme differences. The only way to find out would be to do specific comparison that involved a procedure that removed as many variables as possible. but even then, the results are only valid for that particular procedure. If one supercharger gets 3 MPG better in the comparison, the same supercharger could just as easily get 3MPG worse for your particular driving patterns/habits. And you don't even know what your new driving habits will be with the additional power.