The only way the linkage, be it hydraulic or mechanical, can cause slipping is if it is pressing on the pressure plate fingers. This is near impossible with a hydraulic system. I don't know about your car, but on the 123's there is an eccentric at the push rod where it mounts to the pedal lever. This is only there so that you can adjust to ensure that the cylinder rests such that the piston is at the top so that the port is open to the reservoir fluid supply.

I fully expect that the problem is not hydraulic. Air in the system will cause the pedal to be low and prevent the clutch from fully releasing.

Slipping is typically caused by oil on the clutch faces. This is typically the result of a rear main seal or input shaft seal leak. If this is the case, the gauge measurement doesn't matter. The gauge is indicating clutch plate thickness. Regardless how thick the clutch is, it can slip if oilly. Also weak pressure plate springs can cause slippage.

I have never seen slippage caused by a weak pressure plate except in a car that has been driven abusively.

Good luck,