The only thing I notice when I hit the S button on my XK8 is my wife's nails digging into my forearm.

 

This is absolutely correct. Someone also asked if leaving the 8 in "sport" would harm anything,......the answer is "no". I drove mine in Sport only, for the entire time of ownership. Seems like a lot of people are confused what this button actually does. As it was said before,........nothing except changing the shift points. In plain English,......it lets EACH gear rev up a little higher (in both directions, up and down!) before it shifts to the next gear (up OR down). That's why some of you noticed "better engine braking" with Sport on. If you want to confirm this, ........take off in the 8 (not the R!) nice and easy (no flooring), and observe the tach when you feel the tranny go to second. Stop, hit the "sport", and take off in the same fashion (nice and easy), and you WILL see that the tach is higher when the second gear clicks in.

No effect on steering, suspension, or ANYTHING else. IF your driving habbits change with sport button engaged (you start flooring it on take off, etc.) your MPG will change, and that's it Folks! We can now put this to rest, huh?

 

No it doesn't, any back pressure reduces exhaust flow speed and reduces scavenging and efficient removal of exhaust gasses out of the cylinders.