That's funny because when I dyno'd my engine I gained 26 hp from a header swap, 2 inch to 2 inch / 2-1/8 step headers ....then I wanted to run the exhaust as it would be in the car, so we removed the dyno exhaust....it picked up another 25 hp.... So, that's a 51 HP pick up total.
There was no messing with anything except for removing a restriction.
I'll be putting this engine back on the Dyno soon if you need proof.