i used to run a 3.4kg ultra light JUN flywheel on my 1.3 slx with janspeed system and made a huge difference in the response n improved drivability/feel of the car.
when i first started her it was like a motorcycle 
absolutely helped improve heel & toeing.
I drove her daily and tbh it idled just as good as an 8kg stock flywheel.
the rapid engine response will allow faster upshift/downshift time dependant on the condition of the synchros and possibly faster acceleration cos of both reduced static & inertia weight and more of the available power produced can be used to drive the car forward rather than spinning a heavy flywheel.
possibly downside of a thinner lighter flywheel (prob only applicable to huge torquey forced induction setups) is reduced heat capacity for the clutch making it more likely to overheat & glaze the clutch leading to slip at peak torque if its abused alot such as on dragstrips, but that was prob cos i was running 160ftlb 160bhp turbo setup and twas my first time on a dragstrip with poor clutch control and partially bedded paddle clutch disc.
note about these ultra lightened flywheels with the big gap holes around the clutch face is that they will restrict the size of clutch disc you can use upto 184mm
