This will give you a good rundown on the inner workings of the WUR:
Porsche 928 Forums dot com - View topic - CIS Warm Up Regulator Made Simple
Sounds like the WUR is working ok cold then giving you a lean mixture when the engine warms up, kinda confirmed with the on the high side control pressure reading you took. Warm control pressure should be between 2.8 and 3.2 bar w/o vacuum and between 3.4 and 3.8 with vacuum.
Cold control pressure will vary and depend on the temperature, but will always be less than warm control pressure. There's a chart in the WSM that gives the info (page 25-6).
Verify your warm control pressures and go from there. Remember, the higher the control pressure the leaner the mixture.