Have not tried the Jun flywheel, but most of the light aluminium fly wheels are produced by the same company. Such as Addco being one of the bigest makers of anti-roll bars. I myself am sold on JWT aluminium flywheel. It weighs in at about 8-9lbs. I think the factory one is about 23..???? I have my factory one around here some where. After 5 clutches I found a combonation that works for me and my street/autocross lifestyle. This combo should have no problem suporting my 10psi turbo install. The list is as follows.... Factory, Dual Friction "crap" force, Pacific Performance 3 puck "track only" clutch, Broward clutch 9 puck "junk slipping nightmare" and my all time favorite, that I loooooove, is the ACT street clutch with a full face Exedy sprung disc and 42% over pressure plate good for 236lb/ft. This combo is a no lose setup. JWT flywheel ($500) and ACT street ($250 special price) clutch. Use only Nissan throw out bearing. Any other questions just e-mail.
I have the jun flywheel with an ACT clutch and upgraded disc....great combo! Takes off easier just putting around, works great at the dragstrip to modulate wheelspin off launch, and works good at track days where you need to pull good coming out of turns. The only drawback to me is that it's easier to stall. no biggie. The jun flywheel is made from forged chromoly steel. It's supposed to be 200% stronger than the stock steel flywheel. I bought the 10.5 lb. flywheel. The ultralight is 9.5 lb. Factory flywheel is 18.5 lb.
I am happy with the aluminium one myself. The heat sink properties of aluminium are much better then any steel or cast iron. I know when cast iron becomes fatiqued it will separate with exsplosive like properties. Pretty sure chromoly steel would not have this problem. I'll have to call Mosler (south FL.) and ask. The cromoly, lb for lb, is thinner then then aluminium. It's a tough call, but either way you go i'm sure you will be happy with the lighter flywheel. Looks like we all are.