Your driving style has a lot to do with the mileage.
Try to get off slowly from full stops and not let engine RPMs go beyond 3K.
You can manually try to up-shift the car at lower speed points. This typically reduces acceleration but you get better mileage. This is especially true when you are cruising city streets and not going above 30-40mph. Though you are not going to go above that speed due to speed limits, the automatic tranny is unlikely to go into the highest gear. The program which controls the steptronic tranny knows when it is not safe to up-shift so you are not going to hurt the drive-train by up-shifting too soon.
When you are doing highway driving, ease of the gas once you reach cruising speed. Many times we keep up the pressure on the gas pedal, and gradually the speed creeps up. Avoid the up-creep by easing off the pedal, especially when you are going downhill.
So you do not have to reduce the horse power of the car to get better mileage; just modify your daily driving style. That way you can let it rip when you are in the mood, instead of having a permanently crippled car.