The 616 flywheel is already some 10-odd lbs lighter than the 617 FW. Supposedly, I haven't seen anyone weigh the two yet.

The idle is fine with the 616FW so I bet you could shave more off without problems. There is one video of an engine idling with just the auto flex plate (About 1/4 the normal mass without the torque converter and fluid).

You want some weight or you'll get the engine returning to idle too quickly between shifts and you'll wear out the clutch quicker.