I haven't looked at their claims, but strictly speaking it isn't impossible that adding cargo increases efficiency. How? Well, if the cargo is added in a ___location that moves the aircraft's center of gravity closer to the aircraft's neutral point (the center of lift for the entire aircraft), then less trim force from the elevator will be necessary for level flight, resulting in reduced trim drag.

I'm not saying that their claims are correct, just that added weight can potentially result in improved performance, depending on how much weight it is and where the weight is placed.

Also, regarding scaling results to model aircraft, the subject is called "dynamic scaling," and it's actually a complex task to scale the various model parameters to get good results. Take a look here: http://adg.stanford.edu/aa208/dynamics/nondimen.html, under section 2.3.4, for the punchline. Also, an interesting history of dynamically scaled models can be found here: http://www.nasa.gov/pdf/601262main_ModelingFlight-ebook.pdf