Back on topic
Think of an NA engine like an air pump. adding fuel is easy - you just increase duty cycle and injector size as required, the fuel system is pressurised electrically by a pump so remove this line of thought.
What you need to do is increase the amount of air flowing through the engine. Power= fuel+air+spark. To this you need to increase the valve open period and or the draw of the piston stroke ( compression).
Now all things being equal when you change throttle bodys, manifolds, intakes etc all you are doing is making a less retrictive path for the air to flow along but the main restriction behind all this is piston bore size and compression.
The person above got 10BHP from a GruppeM and tune. A set of cams might give him 10BHP now as well, where as on the stock airbox he might only have got 5bhp.
At bolt on levels you are only removing restrictions but together they add up and even small gains on these engines are very noticable. Tuning is a must though to extract full potential
To reiterate Lee's post above
Its not the numbers on the graph, its the shape and the area under it