Originally Posted by m3chhawk

No the test is invalid because you have no control. You've tested multiple cars, on multiple dynos, on multiple days, with multiple mod iterations, and you are trying measure improvements that are within the experimental error of temperature and humidity.

Get the car setup with whatever "cheap" exhaust you would like and then all in the same day, without unstrapping the car from the dyno.
Dyno chart 1) Baseline it.
Dyno chart 2) Tune it to whatever level the tuner feels comfortable with. A combination of fuel points, timing, etc..
Dyno chart 3) Bolt your mods on. Base line it.
Dyno chart 4) Tune it to the exact same level of aggressiveness as dyno chart 2.

The differences in 2 and 4 are the impact of the mods. If the mods allow you to run more fuel or timing without knocking? Great! Want to change the exhaust again? Or add an electronic fan in the intake? Or a flux capacitor? Cool, but you have to start at dyno chart 1 again.

But until you do this or something similarly scientific, you don't have ****.