Guys,
I'm still reading up on this stuff but do the OPC people actually call these parts castor mounts! The mounts are for the suspoension A Arm (wishbone), which happens to have an eccentric fixing.
I suggest fixing will affect Camber and Toe significant more than Castor (if any). For race applications, extreme adjustment of this fixing (along with a decrease in ride height) will give 3 - 3.5 deg of -ve camber.
The 944 gives a significant range of suspension geometry adjustment, the downside is that it is therefore prudent to have it checked frequently, preferably when the tyres are new, because 944 and 911's do go out of adjustment and a correctly set up car will run surprisingly well!!
andy