Here is a link to an independent end all test of aftermarket headers.
http://www.mygokart.com/exhaust_basics.htm
please note
Some people are not understanding the power gained chart. Those power gain numbers are averages. That means that various hp increases are averaged across the stated range of rpm. You cannot compare these numbers to the peak hp gains that manufacturers claim. The hp gains they report are the max hp gained at a specific rpm. So you can gain a max 15 hp at a certain rpm, but when averaged with lower hp gains of all other rpms the number lessens. So… when using the chart, don’t compare those averaged gains to manufacturers’ report max gains.