It cracks me up that people drag race 3600-3700 lb Audis. Can you imagine the stress that launching an S4 hard would cause? The car is pretty overbuilt (compared to, say, a Subaru) but c'mon, a dozen or so 13 second runs and you'll be replacing everything from the flywheel to the lug bolts.
I prefer to focus on the monsterous 50-150 accelleration. I got some magazine in-gear times from a Boxster S and did some informal comparisons on deserted highway in northern Vermont last summer. The S4 was about 30% quicker--that's right, 3 out of 10 seconds!) than a Boxster S at highway speeds and above.