OK lets have a scenario . . . imagine you are drinking from a cup using a straw . . u suck from the straw to 'pull' up the water correct ? ? ? now imagine the water are the INTAKE air/fuel mixture and your mouth as the exhaust gas . . you need the exhaust system and some back pressure (but dunnoe how much) to pull the intake mixture in from the intake vale during high RPM cam overlap . . . so a good exhaust system not only helps to expel exhaust gas . . it may help in your intake too . . or you can also see it in the way that . . each pulse of exhaust gas from a cylinder . . will help to pull out another pulse of exhaust from the next cylinder . . soO i guess with a well selected exhaust . . you will make more power then without anything and also with lesser noise and pollution

heehee please do not blame me for my rubbish this is how i feel