From what I remember a lighter flywheel allows more hp at top end, and less torque at low end. It doesnt make either, just lets the engine act differently. Without the rotating mass of the larger flywheel you lose that extra momentum. Lighter flywheel allows for higher faster rev of the engine. In the past I have used several lighter v8 flywheels and I didnt like any of them. Maybe it will be more beneficial to a I4 Turbo though.
-Mike-