If I get an X5M we should arrange a showdown! Lol I'm confident if someone did the same modifications you've done your cayenne to that damn x5m that the BMW would be faster. My cayenne which was in many respects similar to yours had a tough time hanging with an x5m that only had a generic off the shelf tune...

 

This is kind of the ultimate question when you're going in to purchase a <100k SUV; loaded Cayenne S or the X5M? I was in this dilemma when I was in the market about a year at 1 week ago for a SUV and I ended up with a loaded Cayenne S for the following reasons.

On Paper I think people have to be crazy to be passing up the X5m's. The only two things that the cayenne has over the x5m is the fuel economy and the weight on paper. When I was shopping around the X5m's were all fully loaded and the dealers were willing to deal at least 7% off the msrp. For a contender that stacks it chips right up to a cayenne turbo i'd have to consider that to be a great value for a german luxury automobile.

However, what's not on paper is the aesthetics of the vehicle, looks, feel, and age. The cayenne feels properly engineered all around. The input of steering and acceleration are very direct, no play. The ride is acceptable and the vehicle behavior can be transformed with a touch of the sport button (the only sport button that I actually like). I got the espresso and cognac interior so the interior is better than anything the X5m can offer and the exterior isn't the warmest but it does feel more "newer" than the x5m's. The x5m does speed very well, it is extremely quick and the cayenne cannot keep up with it. But to me the x5m felt vague compared to the Cayenne. It is what basically won me over.

However the two vehicles are excellent choices. You can't really go wrong with either but in my situation I chose precision over speed and there is nothing wrong with choosing the latter.

 

Thanks Steve for expressing my exact same feelings, when I was in the market for a Cayenne S and an X5m, I'm like the x5m, especially all magazines compare the x5m with cayenne turbo S and still beats the turbo S. the X5m had $9000 off MSRP due to slow sales. and the least payment (money factor) is extreme LOW, BMW financial require a credit score of 640 or higher to qualify, my Porsche sales told me Porsche financial is the hardest, he had clients from MB qualify for $100K loan for an AMG, and he can't qualify to loan a $70K porsche.
So, I went and test drive the X5m, disappoints, it's totally different than what the magazine says, perhaps I didn't use the sport button???!!! it's not fast to start, and sudden acceleration (which is the most important for me when I want to pass or beat the traffic light) is very bull. So I switch over to my wife and she drives it, she told me the same thing. (she drove the new cayenne s and V6, she also test drove x5 5.0 and x5 5.0 with the M performance package which adds 40 HP) and she said the x5 5.0 is better than the X5m.
after I got home, the more I think about the it more I could believe it, perhaps I had too high expectation for the car? a week later I went to another dealer test drive another NEW x5m with my friend who had a x5 4.4, I drove it first and this time I spend more time and attention at the car, it requires a heavy foot for power, cornering has lots of toss and turn. I DO like the heads up display, I DO like the surrounding camera view. My friend is disappoint like I do.

 

I drove an X5M the other day and acceleration is at least on par with my '09 Turbo s, if not better, with more useable power at the bottom end... But to address the comments of others, having the "M" button engaged on the steering makes a world of difference... The sport button on my cayenne(s) always seemed to really sharpen things up but never to the extent which the BMW changes between that button being on vs off. The ride in the BMW is way firmer than the cayenne (turbo or turbo s) but I really enjoyed the sporting nature of that car.