A stiff clutch is often a sign that the release bearing is becoming gummed up with clutch dust, and will need to be replaced at some point down the road. It can also be a sign of a dry clutch cable if the action starts to feel gritty. Here's Steve Reynolds' experience with lubricating his clutch cable:

My SHO has had a vibration sound when shifting above 5000 RPMS. Also it started to have a gritty feel to the clutch with a slight increase in stiffness.

In my opinion I did not think it was the clutch (6000 miles) but it felt like a dry cable. So I used some silicone grease (Dow Corning #44) and lubed the end of the clutch cable as it comes thru of the firewall into the interior. Easier said than done. It would have really been helpful if had a second elbow on my forearm to try to get in there. The real problem is not as much getting in there as it is to be in there and manage to push the clutch pedal down to get to the part of the cable that needs to be lubed.

I also lubed the end of the long white switch rod (for the starter disable) around it's pivot point and up the shaft.

This action has almost totally removed the high RPM vibration sound and after 2 days of working it in the clutch is silky smooth again. I plan on lubing the cable several more times to get the lube to work its way further into the cable.

Maybe looking into trying some lube might be a cheap thing to do before condeming the clutch.