Most of the larger auto mags have run tests with no gain results. The theory is good as far as swirling air is more compressed air, but this is lost in reality. By the time the air gets to the valves, the swirling, compressed effect is lost. If you were swirling it directly before the cylinder (without the interference of the valve), it would work like a champ, but this isn't the case in any engine.
So, it produces no gain. Now, it is an object which sits in the intake tube. This means it is a restriction to the airflow. It takes the diameter of the airtube and reduces it. Therefore, it is the equivalent of having a smaller intake tube. Therefore, in theory, it should reduce engine performance. Does it? I have no idea. I have never known anyone to buy one.
Robb