The reason you will never have definitive data is because it is such a pain to install a header and get comparable results for same car / same day / same dyno.

I have the Mugen header / exhaust (and other minor things) and show more power in a car comparison that indicates good results.. but proves nothing:

https://www.s2ki.com/forums/showthread.php?...&highlight=dyno

 

Definitely Mugen headers because of all the above mentioned reasons especially the midrange gains (really noticeable!!). This is certainly my favorite mod so far..

 

I really like the spoon header I have on my S. I think it made a noticeable increase in power.

 

I really like the driveability of the car with the Mugen header installed, but I'm having a fitment problem. The header changed the angle of the cat and brought it a few millimeters closer to the notch in the underbody for the exhaust and now it knocks against it whenever I go over bumps
This must be rare though, since I did a search and I'm the only one who has ever reported this problem.

 

TODA!

 

Mugen

 

The Mugen adds noticeable bump in torque from 4000 to 6000 RPM. At times I would like it to be louder, but this is as loud as I can live with. It cost a lot, but is still my favorite mod!