I think its a placebo effect, it sounds more powerful so you think it is more powerful. I noticed now that the weather is cooler my mileage was starting to decrease, I put the resonator tube back on and it is now back where it was. I don't know why unless the cold air made the computer add more gas to compensate.
Now I did see some difference when I put a K&N filter in the stock air box, but it was minimal.