properly designed headers will certainly boost power over OEM cast exhaust manifolds
but without a very low restriction exhaust system, mounted behind the headers,
the difference will not be as obvious.
just a bit of info,
decades ago,
I built a 454 high compression, rectangular port engine using a
chevy solid lifter cam _3959180---
https://outlawspeed.com.au/shop/cr131141
11.4:1 compression,
dual quad intake with two 50o cfm carbs, 4.11:1 rear gears a muncie 4 speed,
with stock exhaust manifolds and a stock 1966 exhaust the car ran low 12 second 1/4 mile times
swapping to a 3: dual exhaust with an ( 3" X_ pipe followed by a second 3" x-pipe and dual 3"in and dual 2.5" out mufflers and hooker headers,
the car felt and sounded different, and ran mid 11 seconds which in about 1970 was reasonably fast, especially considering this was on street tires then available.