Did you buy one? Else how would you REALLY know? It could all be propaganda by the oil industry.
I really know, because "twisting" air through your intake would harm performance. Hell, most intake systems are designed to staright then airflow at the MAF, and many of these "tornado" devices go infront of the MAF, so its doesn't matter anyway. They use the swirling water idea, but thats of course silly and completely unrelated. If anything these things restrict your airflow, therefore dropping how much air is read by the MAF, therefore less fuel is dumped into the system... by that logic, it really might gain you some MPG, lol, never thought of that till now. However, if you really wanted to drop HP at the expense of performance, you could just unplug a couple injectors, LOL.