I've had a Fidanza flywheel (which is <10#, as I recall) on my car since 2002. That included with a somewhat "built" EJ25D (cams, ported heads, etc), and a stock-ish EJ25D (just CAI and header/exhaust), and with both a 5MT and a 6MT. I've never had any issues with it whatsoever. I've had a couple of different clutches on the car with the Fidanza - first a Clutchmasters stage 1, then something else that I can't remember, then the 6MT went in with an OEM Exedy) and I have never had any clutch chatter at all. I still remember that I stalled it the very first time I tried to get the car moving after having the Fidanza installed; after that I've never had an issue getting it moving in the ensuing 11 years. I don't have my maintenance records handy, but that's probably 150,000+ miles of daily driving, road trips, auto-x, drag strip runs, and track events.