I drove a Cayenne Turbo yesterday...quick in a straight line but like others note, it feels heavier and less responsive than the X5. That's really no suprise considering that the Cayenne weighs some 600 lbs more. I would definitely prefer the 4.8is to the Cayenne turbo, although the 4.8is needs a supercharger.
Interior of the turbo I drove was nicer than I expected with leather and a nice LCD screen on the gauges. Still, the exterior is so ugly I just cant go for it.
I also had a Cayenne turbo try to mess with me a few months ago when I was in Mammoth. From a low-speed roll he pulled away slowly...not as quickly as I thought he would. On the other hand, at speed I'm sure he'd have left me for dead. The salesman at the BMW/Porsche dealership yesterday told me that the Cayenne turbo will run door to door with a 911 carrera S at highway speeds...until at least 140. I dont know if this is true, but he told me that was from personal experience.