I was just reading a magazine article where they dynoed various different spark plugs as well as these Direct Hits amplifiers. They're made by Splitfire and sit between the plug and the plug wire. They actually gave about a 3 or 4kw increase on top of the best performing spark plug. The runs were made back to back with conditions kept as similar as possible.

It was also interesting to note that the three different platinum plugs they tried gave the lowest HP numbers.