I can tell you in my case it was the pressure plate. My issue manifested itself as a pedal that would not come back up from the floor at high rpm, and would take a few minutes to return to normal. Finally at the track for the second time I got a clue that put it all together. Fluid burped out of the reservoir after it got hot after a few passes (1/4 mile). What is happening is either a geometry problem or a design issue causes the pressure plate to push back on the throw-out bearing while the plate is still clamping on the disc at high rpm. This in turn forces the fluid back into the reservoir (there is no check valve and won't work with one), so when you now press on the clutch to shift, the helper spring holds the pedal on the floor as there is no fluid left in the throw-out bearing to counteract it, and you get no or not enough disengagement to shift. You cant shift if you cant disengage, so you break your syncros if you force it. I changed my pressure plate and disc only (not throw-out bearing) to exedy, and now I can speed shift at 8200 like clock work. The difference isn't a little bit, it is a tremendous difference, so much so that it is undoubtedly the pressure plate. Don't waste your time on anything else. Don't put a ford clutch back in the car. Oh, I'm still on the stock shifter too. My car with a tune and full exhaust runs 11.8 at 119 in the heat with a terrible 1.85 60'. I did this at 1000 miles and I'm still on the original trans. No grinding or other issues.
As a caveat, you will see this same pedal response when you have to pump up the throwout bearing after the clutch change, it will stay on the floor until it fills with fluid.