The EPA has tested many aftermarket bolt-on parts, NONE has increased the mileage by more than 3%. 3% is their threshold for measurement certainty.
The EPA would nearly mandate a company's aftermarket technology if they could prove that consumers would see a REAL mpg improvement of more than 3% (13 mpg x 3% = 0.4 mpg). I heard this directly from Sid at CARB (those in the industry know exactly who Sid is).
If anybody tells you that their aftermarket part/invention can get more than, in this case, 0.4mpg, then either they will soon be multi-multi-billionaires because the EPA will mandate their invention on all production vehicles or they are merely spreading customers' "rumors" that have zero scientific basis.