the differential, you will know if that goes u will know, trust me, it wont be a slipping clutch feeling. Are you doing the install? if so make sure all the bolts are torqued down properly, my friend has a boosted civic and when he upgraded the clutch it started slipping terribly, he bought a new clutch thinking the other one he just bought was crap, but come to find out the clutch wasn't bolted down tight or had come loose and slipping when he got into boost. So check that if you are the one doing the work, if its not then its possible the flywheel is gone, We normally replace flywheels when we replace clutches just to be on the safe side so I don't have any experience with too thin of flywheels.
Hopefully someone else will have more to add, but my suggestion is make sure the clutch is bolted correctly to the motor. good luck.
