The flywheel's weight is most important in drag racing, where acceleration is paramount. A light flywheel requires less engine torque to quickly spin it up to high rpm, so in a light car it helps lower your ET. Conversely, in a heavy car a heavy flywheel helps launch the car off the line by storing energy and dumping that energy into accelerating the car off the line when you pop the clutch. The heavy flywheel is spun up to high RPM at the line just before the 'tree starts counting down. In a heavy car there is less loss of acceleration due to spin it up again under power that it would be in a light car.
In LSR there is plenty of time to spin up the RPM so the question of a light/heavy flywheel isn't too important.
Regards, Neil Tucson, AZ