Scratch the diesel then, as it wasn't introduced to North America until 2013. That leaves either the 3.2 six banger (the Volkswagen VR6) or their own 4.5l V8. The V8 can be had in several configurations that were rated at 335 and 514 hp. I say "were" because most engines lose horsepower and torque over time. Especially those that have been pounded hard.
I personally wouldn't have the six banger, it's the same engine that VW puts in the Golf R32. The Cayenne is quite a few pounds heavier than the Golf. Of the V8s, it sorta depends upon how much money you're willing to part with. More hp = more $