Very nice car! Exactly the same switch I did a year a ago (although mine is a saloon) and wouldn't go back.
Assuming the Shark software is still installed, what's the performance like - Real world driving and actual figures?
Well I've just fitted some roofbars to my S4, finished fitting and locked the car and 2 minutes later the alarm goes off.
Has this happened to anyone else? The roof bars must be part of the cause as this is the first time the alarm has even gone off in the year I've had the car.
I would make the diff the be all and end all personally, drove both before buying my S4 and would say there's a difference but you'd struggle to notice on normal roads (legally).
Your spec is similar to my car minus the sports diff, good choice! Mine might go up for sale if I find the right RS4.