blah blah blah

The flywheel is the only thing mentioned that will really affect how fast the engine rpms fall with the clutch in. It will also allow the motor to rev faster with the clutch in (or in neutral). As people have mentioned, the flywheel's diameter and weight contribute to it's inertia. For those of you who haven't made it to the 10th grade yet, think of inertia as anti-acceleration; it tries to keep things from accelerating and decelerating. It's like momentum, but rotationally. So with less inertia holding the motor back, it will rev faster. Also with less inertia, it will lose rpm faster. This is all in neutral / clutch-in.

A driveshaft has no bearing whatsoever on how fast the engine loses rpms in neutral / clutch-in. In fact, it's not even connected to the engine in these conditions. A lightened driveshaft will decrease inertia in the drivetrain, making it more efficient. You may see an increase in whp even though you haven't increased the actual engine output. This is part of what we call drivetrain loss.

Someone mentioned a crank pulley. A lightened pulley may have some small effect on how fast the engine changes speed in neutral. Not really anything to write home about. Crank pulleys are more to reduce drivetrain loss from powering the accessories (underdrive pulleys only).

And someone questioned the durability of a carbon driveshaft... Every 350Z 6-speed on the road right now has a carbon driveshaft from the factory. They seem pretty durable to me.

Bottom line: an 11 lbs flywheel, a carbon/aluminum/whatever driveshaft, a crank pulley... all of that crap on one motor and it won't drop RPMs too fast.