First, a lot of vehicles on the road can get 20% mileage improvement with a simple tune up, especially if they were made before the ECUs got really sophisticated. Most of the magic devices on the market find this is all they need to get a few honest yet glowing testimonials.
Second, 20 MPG is good for a 6 liter engine, but it's not spectacular.
Third, some of the magic devices fool the engine controls, improving mileage at the cost of lower horsepower and/or greater emissions. Has this vehicle been tested for emissions since the modifications?
Finally, for many drivers (including myself), a 20% improvement can easily be gained with a little block of wood under the accelerator, keeping it off the floor. I suppose a clever limit to the fuel flow or pressure would accomplish the same thing.