So the Cayenne carries an additional 635 pounds and has 50 less horses and 74 less pound-feet of torque. The acceleration battle is easily the E63's with a firstname.lastname@example.org 1/4 mile versus the email@example.com for the Cayenne Turbo. Very fast for an SUV but the E63 is simply ballistic for a wagon.
The extra weight and higher center of gravity equate to a 1:47.09 laptime around the track for the Cayenne Turbo versus 1:44.72 for the E63 Wagon. Again, a night and day difference.
Braking performance is close and the Cayenne actually manages to pull more on the skidpad with .91g versus .89g for the wagon. So the Cayenne Turbo is no slouch but the E63 AMG wagon is simply too much to overcome. Additionally, the E63 AMG has rear wheel drive and not all wheel drive so not exactly 100% fair when attempting to state the wagon makes more sense. That all changes when the E63 AMG gets 4Matic all wheel drive in its facelift though along with more power.
So yes, a performance wagon can definitely replace an SUV and actually makes more sense fro ma performance standpoint. Road and Track agrees, full article below.