I think you have to consider whether the head loss is on the suction side or the pressure side. If you add loss on the suction side (close the suction line), you will reduce flow and reduce the filter pressure. Alternatively, if you lower the head loss on the suction side (like the OP did by going from a hose to PVC with skimmer), the flow rate and the filter pressure will increase.
If you were to add head loss on the pressure side (say the filter getting dirty), then the flow rate will drop and the filter pressure will increase.
