I bough the car with a stock flywheel in, and changed to a lightened one, got sick of it. As Duncan says driving around town its annoying. Anything under 2,000rpm and my car would shudder... I changed back to a stock flywheel and am glad i did. Car is much nicer to drive with the stock flywheel.
I never noticed a speed increase in reving, but that would probably because i fitted a twinplate with my lightened flywheel, so the twin plate+lightened flywheel probably weighed the same as the single+stock flywheel.
My advice? Save your money, keep the stock flywheel (get it machined if it needs it obviously). Jim Berry told me the same thing - said the Japs usualy get it right with thier flywheels, they are a good weight. Being turbo cars we need to keep the rpm spooling, if it was a torque monstor down low, then maybe consider a lightened flywheel.
Hope this helps.