I just recently purchased a 20 P300 R Dynamic, and my first impression was the lack of a tuned exhaust.
Due to limited access to a Jag Performance system, I had a custom exhaust shop look into it and they just removed the massive original muffler and replaced it with a nice stainless Hi Flow muffler from Vibrant Performance, its no where near "loud", but now has such a great deep tone throughout the rev range, especially at high revs...….maybe its just in my head, but it seems the performance improved as well.