There are videos out there, two that I remember were from television stations, using their own fleet vehicles... like the news vans that they drive like 100,000 miles per year.
One in Florida
(Full size Ford Econoline with 460 V8 I think) saw a decrease in average fuel economy by about 2 MPG. This was over a 10,000 mile test I believe.
The one in Ohio or Pennsylvania gained 1 mpg average over a shorter test, I believe it was the duration of an oil change... 3,500 miles or so. This was a Jeep Grand Cherokee with a 318 V8. Their follow up report said that they installed a Tornado in a Chevy van and lost 3 MPG...
If these things worked with any predictable reliability or genuine measurable increase in HP or MPG... Well, they would have names on them like Mr. Gasket, Edelbrock, Holley, etc. All of the big name manufacturers would be getting into the act and trying to sell their own version.
But they don't... (as far as I am aware) because they won't risk the names of their companies for a quick buck gained from screwing over decent people out of a hard earned dollar.