Ummm, If you can understand the rantings of a "Fog Breathing Limey Brit B'Stard" I shall try to give you the briefest details ok?
Ok, the "Jetting" you refer to is actually to do with the fuel system-not the pipes...lol...by changing the pipes you are creating a different back pressure from the engine through the exhaust pipes ok? What "Jetting" does is to change the idle and main jets in the carburettors to balance the flow and back pressure to aid the smooth running of your engine...It may run ok with aftermarket pipes if they are directly made for your make and model, but most are general pipes and it is best to re-jet the carburettors...If you are used to working on carburettors then its no big deal - but, if you are not confident with carburettors then get a pro to do it ok? Make sure its someone who understands your msake and model and the principals of the correct back pressure etc...hope this helps mate...