I could possibly (only very very slighly) understand the idle appearing to be differant but ONLY if your using a flywheel with a smaller diameter i.e for a 5.5" clutch for instance, but a std diameter cosworth flywheel with a std size clutch - if lightening your flywheel has made your idle speed lumpy then theres something wrong.
iirc the std 4x4 flywheel is something like 13kg ? my flywheel with ring gear is 3.5kg and i use a triple plate 7.5" clutch, ive never noticed any differance in idle quality at all.
and the gains from it are massive, the whole engine feels smooth and that it wants to make power and get going, unlike most YB's that are very lethargic.
I would say that you have to have 'better'clutch control than with a large flywheel as it makes that car easier to stall, this is where the stored energy at slow speeds (only take up) is helpful, but ive never stalled mine in proper use.