I don't believe that is correct statement. In reality, both sides of the motor are very restricted, the intake and the exhaust. You will get the greatest benefit from a combination of ramair/K&N, plus headers and free flow mufflers and of course custom tune. There will be a gain from doing ramair/K&N and free flow mufflers, plus tune as well, but obviously less than complete exhaust. Based on personal experience, ramair and free flow mufflers plus tune most definitely deliver a difference. Just doing either free flow mufflers or ramair alone provides little gain. If you want rip the knob off power, headers is the way to go 
I just haven't been able to convince the misses that giving Paul Bryant a sizable amount of cashola for headers and complete exhaust system is a solid investment. Women...
