in my opinion, i think a well tuned exhaust system, (not that any of us can change our exhaust everytime we make a dyno run), will yield the greatest gains because when both of my tail pipes rotted off the muffler, my truck ran like ****, besides sounding loud and hollow. i actually went under the truck and welded one of 2 exits shut. after that, it felt torqueyer and sounded a little better too.
i think that a certain level of ''back pressure to hp'' is very crucial for maximum hp and torque. just my opinion. theres probably some science
behind it, im just not smart enough to know what it is.