HEAVY clutch

Look into the cable. If a cable is too stretched out, the pedal can get incredibly heavy. Also, I believe if the cable is incorrectly setup, it can cause slippage - I'm sure someone will chime in with more input though.

Jeff's got some good thoughts there.

Where is the point of engagement? If near the top, that can lead to slippage.

Too tight of a loop in the cable can create the tough pedal effort he mentioned.

Is the cable centered in the hole in the firewall? If it's off to a side (bad quadrant alignment), that can cause a brutal pedal.

What kind of mods are done to the car? Is the crankcase evacuation system adequate? If not, the RMS often starts passing oil, which contaminates the friction surfaces.