Lightweight flywheels and spur gears have their ups and downs, it really depends on the terrain you are running on.
The up to a lightweight, is that you will be able to spin up to full speed much quicker than normal, since there is not as much weight for the motor to push.
Yet at the same time, there is also going to be a reduction in torque.
Now granted I do not have the expierence with this in RC cars, but the above stands true for real cars. I would assume that logically the same would stand for nitro RC as well.
I think it would really just depend on the application.