Thats very true. Honda got 40 more hp from the header/exhaust redisign. The most we could hope for as aftermarket bolts is maybe 20-25. Could you potentially hinder the hp gains by doing an aftermarket exhaust?

 

You may get a few more HP, but you probably will lose low end torque. That would make the car slower off the line.
For a 4cyl the torque loss would be greater and the HP gain would be minimal (if anything.) Not 100% sure about a V6.
At Summit Racing, they revamped a 5.0 mustang and showed the dyno changes before and after certain mods. I found this very interesting.
After adding basic bolt ons: Upgraded ignition, plug wires, cap, underdrive pulleys, cold air intake, mass airflow sensor, & exhaust; the only gains were 11.7 hp and 5.5 ft lbs torque at the wheels. And that is on an already powerful V8. It just goes to show you that bolt ons dont add even close to the amount of power the manufacturers claim.