I changed out the headers on my SPF/Roush 427IR, and also changed out the SPF packs with the Classic Chambered Cobra Packs. I followed with interest numerous discussions on the site where claims were made as to the changes in performance associated with these modifications. I wish I would have dyno'd the car before and after but I did not have the opportunity. The changes made a Huge difference in the performance of the car. The noise level is appreciably higher when accelerating but at normal cruise levels I find very little difference. A number of cars in my area are making the same modifications due to the performance changes witnessed in my SPF. Working with Classic Chambered and Reno Engine and Drivetrain (RED) was great and I highly recommend them.
www.classicchambered.com