The 911 is one of the few cars that have clear access to the exhaust system and header. While not the easiest mod to make, headers and exhaust can produce the most HP gains (15HP to 25HP), especially up and down the rpm curve. Headers can come catted or de-catted depending on your needs.
Headers and/or exhaust will have the largest impact on your 911 - it will get louder and meaner sounding based on what parts you choose. Some exhausts are milder than others. Headers will typically sound louder and can have a slight ‘ringing’ sound. The best course of action is to select a header and/or exhaust is to listen to as many as you can on YouTube before you buy.
The point here is if you like the stock sound of your 911, don’t mess with the exhaust!