I thought I'd chime in on the driveablility of Light flywheels.
Downshifting is sped up due to the quickness with which the engine can build revs. Low gear acceleration is improved and driveline harshness goes down.
Popular perception is that lighter flywheel is harder to start/stop. I disagree with that statement. Perhaps with a "race" type on/off switch clutch that allows for no friction zone that may be the case. Mine was equiped with a uprated but otherwise stock type clutch. I NEVER in my ownership of the car killed it accelerating from a stop.
Daily type starts are.. Rev to 1500ish, get into friction zone, apply increased throttle, release clutch, go. In this environment the light flywheel actually helps. You don't "store" enegry in the flywheel but depend on the engines very quick response to throttle inputs to react through the friction zone. It is only marginally different than driving with a heavy stock clutch. I had no problems adjusting between the 1997 ES miata and the 1996 with the lightened flywheel.
I put one in my Volvo PV544 - lots of you older folks read that without pause, but as a guy turning 40 years old this fall, they are no longer recognized by my peers. Anyway, I had a factory flywheel lightened. I do not to this day know for sure if it is cast or forged, but I know dozens of SCCA ITB cars and at least 2 local IMSA GT3-a-like builds that have used the OE flywheels, drastically lightened and most work. I do recall an ITB guy (Sam Moore, ending up DNF at the nationals, tho). Volvos of that vintage used startlingly good metallurgy. Kevlar blankets exist for the worriers.
Here's what I noticed- 1st and second gear acceleration is remarkably better. This was with ratio (3.13 and 1.99) and a 3.31 rear axle with 24.9" tires, so relatively tall
compared to many, so the effect would be even more
pronounced with say, a BMW e21 or a later Volvo m46 box, both using 4.03:1 first's, a short axle and shorter tires.
Stalling- only troublesome spot was backing out out of parking stalls- some with a slope might be troublesome or at any time when you are still rolling backwards and want to use the clutch to get first gear engaged and move forward. There is no coverage to stop a slow roll and relaunch the car the other way- you end up revving the cr^p out of it, or stalling.
It was also way easier to do smooth shifts, ie smooth clutch engagements. Up and down.
I think I had it dropped from about 20.5 to about 15.5lbs, but most of that came off the outer diameter, so the before after inertia is changed waaay more than proportional.