Start with the easy things: Which filter are you using. If the filter you have is not of a very high quality the volume of
oil going into the filter (as RPM's rise)is retricted by the size of the holes around the perimeter and the filtering media itself. Pressure drop as the engine warms is due to the
oil thinning and to some extent the clearances increasing. Secondly: If the pump is questionable it can easily be changed out, if you are running a remote cooler a hi volume pump is a good idea.