If you want to add more pressure, the HV pump is not the way to do it. Get the MP high pressure spring and put it in the pump you have. But 15 PSI at 800 RPM hot is plenty. If you don't think it is, unscrew the filter while it's running.
Oil pressure is a balance between pump volume and leakage out bearing clearances, lifters, rocker arms. The pressure relief spring is to limit the maximum pressure.
Now if the low oil pressure light is flickering in gear at idle but goes out as the revs come up, good chance on a high mile engine that the bearings are worn. Early overdrive equiped autos had this problem due to low oil flow at city road speeds. The factories went to slightly higher volume pumps to provide the low RPM pressure. Replacing the worn main and rod bearings fixes the problem. Those low oil pressure light switches usually ground about 5 PSI to turn the light on.
New engines are now built for the lighter weight oils and low RPM in mind for fuel economy and bearing life.
