I had a C3 and the 80lb effort on the pedal was a good workout. I believe that I saw somewhere, can't remember where, that the ZR-1 stock pressure plate is also an 80lb effort. I don't know if that is higher than other C4's of the L98/lt1/lt4 options. I would think that any replacement parts would be the same or higher than the stock set up. I know I stepped on the clutch in a friend's C5 and it seemed less leg effort...The stock shifter in my 90 reminds me of my M22/Muncie linkage/C3. At slow rpms it was a bit stiff to move, but if I laid into the gas the shifter got easier to move. My Z seems to behave the same way...lay into it and the shift effort seems easier & smoother. Obviously the zf6 is a rail box(?) vs the external linkage that we come from so I would think the zf would have a different feel? Your stuff may just be "tight" like we used to say back when.
Parts can be had at places like NAPA, autozone, etc...they sell LUK OEM at autozone. Carolina Clutch and DRM & Haibeck sell the trick stuff also...fairly easy to get everything except the fork and forkball. This might be worth reading, ZR51Performance.com, zfdoc.com's site, Bill Boudreau's site...lots of good stuff over there about our clutch/trans/hyd system...great Q & A section and Mr. Boudreau is a stand up guy if you need to e-mail a Q he will answer!
You don't hear any grinding when you shift? You don't feel anything when you shift...like the shifter hits something for an instant? Is it hard to get into reverse at cold start-up? It goes into all the foward gears easily as you drive & shift, like 2nd to 3rd is smooth, right? You check the clutch fluid & it's not vanishing in small amounts? I've never had a clutch slip so I can't speak to that. I remember back when, 70's, most mechanics would try to start off in 2nd and see if the car protested...I don't know how valid that method is.
Tom