From my research to maximize the performance gains you need to get as much air in and then out as efficiently as possible. If you only do intake or exhaust you are only improving only half of what you need to do. For example replacing the intake you will improve the air coming in but if your exhaust has a lot of restriction the pistons have to work harder to force the gases out.
Headers improve scavenging and back pressure. Scavenging ensures that there is no destructive interference between the exhaust flowing from one cylinder to another. In-fact good scavenging causes a low pressure region to be created downstream of the exhaust port of the adjacent exhaust pipe sucking the exhaust gases on its own. This is usually accomplished by tuned header systems where all the pipes are of equal length and designed to optimize scavenging effect. Most headers that you buy are not tuned headers, an example are the Hooker Comp and Super Comp headers. The prices vary a few hundred dollars between the two.
Then other factors also come into play. A cam with a higher lift and duration on the intake to get more air/fuel into the cylinders, bigger carb to add the required amount of fuel needed and low restriction mufflers.
Sorry for this being a little long but I would say the headers would only add 10HP if nothing else was changed. Since you have changed the cam, intake, carb, and exhaust already I think that adding headers will probably add more power than the 10hp with just headers alone. Don't think that tuned headers are needed as they are intended for higher performance motors.
I am not an engine builder but I did do a lot of research before I made the changes to my motor. When I was younger I only put headers on my car and really didn't notice any difference and was disappointed. Again just my opinions.