The Cops and a kill AHH!
Most of this "feeling" comes from 2 things -- the Z's very linear torque curve, and the fact that it's drive-by-throttle. So that the only way you're gonna get 100% throttle is if you literally mash the pedal in the Z. This is not so in the M3, where at about 40-60% throttle you're actually opening the throttle by 90% or more. So, the psychological connection makes the M3 seem faster when, in fact, the Z is its equal.
That lack of "pinned back into the seat" acceleration also saddened me about the Z, so as a result I saved up the money and dumped a shitload of it to TT the car. And, by god, what a difference that makes. My motor is generating more than double the horsepower at a mere 9psi, and with the Z's already torquey motor, the feeling is insane. I took a ride in a Lamborghini Diablo 6.0 VT today, and that V12 feels very similar to my Z in boost. Of course, a diablo's a diablo and the Z's just a stepping stone on my way towards owning one. =)
I can't wait for the roads to clear so we can go have some fun

