A long while ago, in my youth, I used to hillclimb a Lotus Sunbeam. This had a 2.2 16v Lotus twincam stroked to 2.5 giving about 220bhp. Eventually settled on a 2.25'' twin box system as power dropped off with anything larger. I am not an engineer but I beleive that it is something to do with retaining some back pressure as opposed to being totally unrestricted. Likewise tuned length of the header tubes can move power up or down the rev band.
if you put another exhaust on than buy a complete or a part that is ben tested on your model because the only cars that benefit from realy big exhaust are turbo cars because they dont have to press the gas away from the turbo and that gives that the turbo get rpm easier .a normal road going tuned car needs a litle back presure othervise the tourqe disapears ,a race car that dont needs tourqe needs very free flow .