Answering my own question, with information I received from the excellent Pelican Parts customer service -- perhaps others will find it useful:
Typically the clutch release/engagement point gets higher (pedal had to be further out) as the clutch plate wears, but that is not the whole story in deciding when the clutch needs to be replaced. Here is a good summary of the decision process from Tony Callas, one of the best Porsche mechanics in the business:
"I don't go just off of the release pont anymore. I just use it as a factor in my decision. The best way to tell is usually the feel of the clutch pedal , how heavy and graunchy it feels. So I would say to use all the following to determine if you need a new clutch. The clutch feel, the sound, the age and mileage, the history IE track use, the shifting feel and of course the release point. I have resurfaced many Dual Mass Flywheels mainly in the 964's but in a 993 it is rare because I am so worried about DME (computer fault) misfire codes from a faulty flywheel so most of the time we replace them. Again there is a whole process in making that decision. One last note, there are many ways to machine flywheels, you have to be very careful on who you use for this operation. The machinist must be very good and use the correct machine."
That said, I think the problem you are experiencing is caused by failure of a roll pin in the pedal cluster. When this pin starts to shear, it allows the clutch pedal to rotate slightly on its shaft. This is a $2.25 part, but a bit fiddly to install, as it requires pretty much full disassembly of the pedal cluster (while you're in there, replace all the bushings and other wear parts).
If you to to this link, scroll down one page, select your model/year, and drill down to main group 7, sub group 02, illustration 702-00, you can see all the parts of the pedal cluster, with the roll pin shown as item #8:
https://www.porsche.com/usa/accessoriesandservices/porscheservice/serviceandorgininalparts/originalparts/