Normally it's always a good idea when one is upgrading the exhaust system to also address the intake. After all the input and output must be matched if performance is he criterion. Thus a stage I would be a good idea. Having said that, there are a number of people who change exhausts for only sound or aesthetics. You decide what is it that you want and of course how deep are your pockets.
From experience in racing any changes to the intake/exhaust really need a change in the cam. Without changing the lift and duration of the valves most intake/exhaust modifications don't do much because the valves opening and closing the primary restriction. Just my take on it of course.