Non of that hurts. Tube headers will never hurt. Free flowing exhaust either. But the long tube headers which look totally cool are designed for camshafts with overlapping valve openings. The way they work is that when the exhaust valve opens the gases go through the tubes with sufficient velocity to create a small amount of momentum and draw a vacuum on the combustion chamber. If the intake is still open a little the vacuum is pulled on the intake too and draws more fuel/air into the combustion chamber.
That being said, headers don't hurt. But I don't think you will get the gains you are hoping for. Something that will work is a hydrogen cell, which is just seperating water into hydrogen and oxygen and routing down the intake. You can make your own, or you can buy kits. Either way, that is the most promising. I am working on my own that will be fairly sophisticated but you can use a simple one in a mason jar with an on/off switch for good results and increased milage. Lots of stuff on YouTube about them