A lighter flywheel allows you to rev up to higher rpms faster between shifts, which can be handy when doing heel-toe shifting.
Some advertisers suggest power increases from installing a lighter flywheel, but such claims can be a little misleading unless you remember that they are only talking about obtaining rpms faster with the clutch depressed (i.e., "rev up as if you had an extra 100 hp!") and not an increase in power to the rear wheels. There are also debates on whether or not a lighter flywheel has a negative impact on torque, but, in my humble opinion, such debates are largely theoretical as I don't think you'll notice any difference either way. I certainly didn't. In short, at least in my experience, other than helping out a bit on heel-toe shifting, there are no other benefits that you'll actually notice.
Personally, in addition to the upgrades you've mentioned, I think a shorter-than-stock shifter, a real roll bar and a racing oil pan are all more important upgrades than a new flywheel.