For me, and perhaps others, this before-and-after would be interesting, because it best represents the potential gain from where we are, to where we "could be" with the spend on the headers.
Begin: A car that already has a cat-back, cold air intake, high flow cats (or test pipes), and an optimized tune
After (1): Remove stock manifolds and tp/hfc, replace with headers. Re-tune for the headers, preferably same day/dyno/conditions (I know the installs are a PITA)
After (2): Do the above, but install a catted y-pipe, for those of us that have to retain cats for emissions or racing rules reasons (SCCA Street Touring where I run)
The reason I didn't pull the trigger on a header is I haven't seen a proper before/after done yet - one where the contribution of just the header can be observed, with all other variables properly controlled.
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