Headers are a benefit only to racers. They have to be uncapped to work. It is based upon wave mechanics from high school physics; so if you run a muffler (required by law on the street, you see little to no benefit.
Tubular headers can (not necessarily) reduce back pressure in the total exhaust system. BUT a crossover tube on a dual exhaust will have a greater impact in terms of adding horse power. This is why every one runs such large tubes on the street. Though to be true most don't know why they run a three inch exhaust system, they are just joining the in fad in following all of their friends. In addition to the sought after louder exhaust (equating noise with power) the larger exhaust size reduces back pressure which enhances cylinder filling on the intake stroke by getting rid of more of the exhaust gasses than a smaller exhaust system would.
I am a degreed engineer; and I studied exhaust gas theory in how it affects an internal combustion engine. Since standing waves used to create a low pressure area out side of the exhaust valve will not work when the header is connected to a exhaust pipe, that leaves only the pulses in the tube caused by the plugs of hot exhaust gas expanding out of the cylinder. So long as the round trip from valve to valve between two adjacent cylinders exceeds eighteen inches you do not have to worry about reversion from your exhaust system. Tube headers accomplish this by isolating the cylinders. A cast iron rams horn manifold does it by the size of the pipe (two and a half inches from port to tail pipe) which offers more room for the exhaust plug to expand into.
This is why a Rams horn manifold makes more power and torque at lower RPM than a set of tubular headers (which are actually more restrictive). You won't read this information in a car magazine because they are paid to advertise (and hence they are trying to sell) tubular headers.
A member here built a 383 SBC and tested headers and cast iron manifolds on his engine in the same day (weather and temperature affect dyno pulls so you can not compare dyno pulls directly unless they are at standard temp and pressure (that is to say the numbers are corrected) which cause people to claim that the comparison isn't valid.
Here are the results of that test:
You will note that it isn't until the engine is spinning above 5,200 RPM that the header makes an appreciable contribution to the power. If you have a hydraulic cam that gives you only a 1,000 RPM where a header makes seven additional horsepower. Is it worth the hassle?