2 psi is not much back pressure Dave.
http://www.onlineconversion.com/pressure.htm

See Bill Schertz charts for the stock Mazda pump.

Paul Lamar

Hi Paul,
this was residual pressure after the water passed through the block,
running at 3000 rpm.
FWIW, Dave McC

It is a smaller pump. There is no magic in 100 year old
centrifugal pumps. They all have about the same efficiency.
This one is designed for a lower HP engine therefore I would
not use it on a rotary aircraft engine.

To quote KSE.

"The higher flow and pressure capability of the H.P.D. water
pump decreases hot spot gasification and creates a higher
degree of boundary layer scrubbing.  This results in improving
the heat conductivity to the coolant."