but what happens when you bring the RPM back to 2300/2200rpm for the climb
If you do that with high MAP (low rpm) you run the risk of detonation. which you will not hear because the engine is not silenced.
The thing that really confuses me is the difference between a CSU/PCM?
They are not the same. The PCM (or PCU) is in the propeller hub, but it is controlled by the CSU which is remote.