I've got a 4.5 kg aluminium flywheel in my MY98. I can't remember what brand it was, Locally made IIRC, been in there about 5 and a half years.

Pros = engine revs faster and spools up faster
Easier heel and toe downchanges, cause you don't have to use too much pressure on the throttle to get it to rev up.

It 'may' help with fuel consumption too. On a recent trip to Kellerberrin I did get about 8.5 l/100 km, which is the best I've ever got. although road conditions may have been a factor too

I have NFI if it's faster cause I haven't taken it to the drags, ever.
Gearchanges flow more smoothly when being driven hard though.
It seems to suit the AHG road circuit and skidpan,

Cons = Gear lash on the overrun, it can sound disturbing until you're used to it.

Lack of midrange torque, you really have to row it through the gears more.

The 'thumping' from the diff on agressive gear changes seems to be more 'violent'

There is quite a bit of driveline shudder when pottering around slowly

That's about it really, I'll edit this post if I remember anything else