It gets affected by inclines more too, as you don't have a stored mass of energy that you do in a heavier flywheel, if too light they can grenade!!
I have run very light flywheels on numerous cars and they do affect idle, depending on how light of course, for road use I would say of what I have had 25% - 30% made a big difference without seriously giving any side effects.
Saying that the cossie one is stupidly heavy to start with, and a lot of the ring can be machined off.
tabetha