Pressure is an indication of two things in a plumbing system, flow rate and head/friction loss. The higher the flow rate, the higher the pressure will be for a given plumbing system. The larger the pump HP, the higher the flow rate and thus the higher the pressure will be.
17-20 PSI is not unusual by any means for a pool. I run at about 19 PSI with a 1 HP Northstar pump (SF = 1.85), 2 1/2" suction plumbing and 2" return plumbing. With the solar system on, the pressure rises to about 27 PSI.
The equipment can usually handle more than 40 PSI but most pumps can't deliver more than about 35 PSI before dead heading so pressure is usually not an issue but a just symptom of the plumbing design.
Also, when you increase the impeller clearance it reduces the pumping the HP of the wet end. Unless you reduced the HP of the motor as well, this reduces the overall efficiency of the pump. You will get less GPM per watt of energy doing that.
However, after you changed the motor, the PSI went back up which is unusual. The pressure is determined by the impeller design and the motor needs to be matched to that so it doesn't over heat. It could be that there was some blockage in the impeller between the second and third steps which cleared after the work was done.
What exactly did you do to the impeller between steps 1 & 2?
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