i've been driving a fit for just over a year now. got a new job at a different shop that does exhaust so i got a straight through muffler for it and welded it on. also took out the resinator so from the cat its factory sized pipe till the muffler which is 2 inch. i just ordered a generic performance header for it. im assuming the size on that is somewhere around 2 inches.

my question is, would upgrading the down pipe, cat and back of cat to 2 inches or so greatly help me? does having this header usless if it cant flow into a bigger pipe? or will it just kill my lowend power?

 

It will help you make a lot of noise!!!
You will also lose some low end power,
Keep you B pipe and get the header.