Here is the way I think about it: The piston goes down, intake valve is open and that creates a vacuum at the carb. This is why the engine sucks in air and gas. You open the throttle and in comes more air and gas. To first order, the vacum your engine makes has nothing to do with your exhaust because most of the time that the sucking is happneing, the exhuast valve is closed. Now, stricktly speaking there is a lot of fluid dynamics going on, some valve overlap, and your exhuast could have a small affect on the amount of vacuum created. Changes such as elevation and temp create a much bigger difference than changes to your exhuast. I think this this explains why minor to moderate exhuast changes have little to do with your jetting. Now, if you changed your cams, or other parts of your engine you could imaging that you might need to re-jet because these would effect the vacuum generated durring the intake stroke.