Great family car, I love my Cayenne Turbo, despite the throttle/shifting delay issue. If you more of a cruiser, this is the right car for you and your family, especially because the high torque and turbo boost provide a great pleasure to drive this car even at lower speeds.
Fuel consumption is a catastrophy in my opinion, my car usually eats around 26 litres/100 km while driving in the city and maybe some country roads. I never got less than 24 litres/100 km lately but I also have to say that I usually drive it pretty hard or as hard as possible in the city...which is more of a stop'n go game.
That said, be prepared for some pretty high fuel bills.
At full speed on the Autobahn, my Cayenne Turbo usually consumes around 28-34 litres/100 km, my highest fuel consumption was in the 38 litres/100 km range when I was driving on a very hot day at top speed for a few minutes.

For comparison: my 997 Carrera S takes around 16 litres/100 km when driving it the same way and on the same routes as the Cayenne Turbo. This might help you to compare it with your own driving style in your 996.