The headers definitely open up the exhaust to the point it helps the engine rev a lot quicker. It gives it a slightly higher pitch vs stock or HFCs. The famous F.I. hiss sounds even more pronounced to me now.
You
do feel seat of the pants gains, but the
total gains won't be realized till you get a tune to incorporate all your mods into the ECU. That's when you'll get the final, complete picture.