Running the Porsche kit front and rear will not ensure proper brake bias. Vehicle weight bias, center of gravity and suspension geometry will all affect the dynamic brake bias. The Z shares none of these aspects with a Cayenne. If you are not already overworking the factory brakes you would be best off replacing the stock rotors and adding performance pads and good brake fluid to increase performance. Larger brakes will only add rotating weight and hurt performance. If you are going to track the car on a road course regularly then look at a properly designed big brake kit. Otherwise, don't kid yourself and admit that the "brake upgrade" is for cosmetics and just pick what looks good to you.