Gains depend on cam profile, head flow, head flow numbers, stroked or not, and dozens of other factors. A stockish type low cam lift stock port engine will have modest gains like you are thinking. A stroked small block ported aluminum heads 600 lift cam ect will be gaining 50-60 or more hp with headers straight through mufflers bigger pipes etc. So its a depends no blanket one size fits all headers will gain this much all the times ect etc etc etc....
A lot of times ppl have say 340 manifolds with stock head pipe thats 2 inch off manifold pinch 1/2 inch around torsion bar then goes into 2 1/4 pipe drivers side. They take all that off swap on headers mandrel bent 2 1/2 pips see a good gain say see headers for the win. Put some 2 1/2 TTI mandrel head pipes on 340 manifolds base line that first then go to headers that a more accurate comparison. But of course most ppl gonna just go gloss over those so called details right???
Then you got stock chrome tips with inner diameter like 1 5/8 choke point where they slide in the pipe. Choke point.
So you can run manifolds on mildish builds with mandrel bent 2 1/2 pipes, straight threw muffles, tips that slide over pipes not into them, and its gonna flow way more then the original exhaust.
