Totally agree about side markers. Most people, regardless of their personal taste agree with this.
The rest might be too pimpin' for folks or not in their budget especially after thowing down the coin on an expensive car like yours but,
Maybe another set of shoes? 20's (Sport Technos or aftermarket) or 22's (all aftermarket). Check the link. Click on the wheel that looks good to you and they will have a few examples mounted on Cayennes.
http://www.wheelenhancement.com/index.php?t=Wheels&ijt=car&id=6Tinting perhaps on driver and passenger side to match the rear sides.
What color is your interior and do you have wood or aluminum trim? Carbon fiber wheel on inside, with a matching shifter looks hot, especially with a titanium gray ride. Only if you have a black or gray interior without any wood. But don't overdo the CF.
Matching dead pedal also finishes out the floor
http://www.eagleday.com/cayenne1.htmlBiggest appearance changes would be clear side markers and larger wheels.
Performance....now that is another thing!
Here is a start -
http://www.evoms.com/Cayenne%20s%20main.htmHope this helps and gets your mod juices flowing!