Interesting, although unfortunate. I have the same clutch in my car, paired with the WRX SMFW and TSK3 and the pedal is light, although with more feedback than the OE setup. I can modulate the clutch through the full stroke off the pedal. HOWEVER, I have driven in customers cars with the same clutch and FW and found the pedal to be overly stiff, relatively "numb", and to only have action at the last 10% of travel. None of the ACT clutches I have driven have had any problems with slipping.
I can't help with a used clutch, but I would be happy to try and save you some cash on a new setup if you need to go that route.