Fx and PCP operating cycle question

Hi all, 

Im currently playing with a FX impact getting my head around the settings and the actual physics and mechanics to understand it. 

In Short I get that more pressure means pushing a pellet faster. 

The hamer sping hits open the valve so more hammerspring tension means the hammer hits harder opening the valve longer letting out more of the predefined pressured air. 

The valve adjustment screws in and out stopping the valve at the end of its travel, this results in "stifling" the valve to either reduce the amount of time its open and or stop it bouncing endresult slight decrease in velocity of pellet the more you screw it in and saving a bit of air if it is slapping. 



What i think im missing is the physics side as in if I use a lower pressure with higher hammerspring tension and valve adjusted ( to give a either a longer shot of low pressure air vs short shot of high pressure air) and how the plenum effects this 



In short I guess in my mind its like if there is oversupply of air it is inneficient as the pellet needs to accelerate down the barrel in x time to reach its velocity ( so a longer barrel gives a pellet more time in it so a lower pressure/ amount of air can achieve the same velocity more efficiently than a shorter barrel?) 

What other factores should Ibe looking at here??