For what it's worth, I added Cobb to my 981 S and purchased the PDK option. I have stock exhaust. I really haven't noticed improved engine behavior but I love what they do to the PDK. Standard mode now shifts much more aggressively and sport mode is pretty crazy - good for track, autocross, and crazy street driving. Shifts are crisper and they've eliminated the dreaded kick-down where even in manual mode the stock PDK would downshift if you quickly put pedal to the metal.