I recall reading something about so called claims of "increased mileage," "more horsepower," "improves fuel economy," etc.
Do you ever see bottles of mystery juice on the shelves of automotive stores that have a claim on them that says something to the effect of, "add 5-7 hp just by using this product?"
The article said most people never see any improvement. And the reason their claims aren't considered false advertising is because what they're really saying is if your engine is performing worse than it originally did, and you use their product, then the product will restore some of the power you lost.
So in order to see any benefit, you'd have to have some sort of loss, which that product can remedy either partially or wholly. But reading the packaging, one would believe you'd get more power on top of the rated power of the engine.
It's been several years since I saw that article, but it's stuck with me all these years.