[Scrappy]

Ok - ya want an answer. Adding a header system improves the exhaust flow to a degree. It could possibly cause a leaner condition in the cylinder as it could feasibly "over scavange" the cylinder on fill (raw air/gas down the exhaust pipe). Since the fuel is port injected and is fixed by throttle opening and ECU timing, additional air could be drawn in and alter the fuel/air mixture. This is really reaching, as the other variables are cam timing, effeciency of the intake tract, etc, but theoritically feasible.