David Leonard

No.  Less back pressure equals flow.  It’s the law for centrifugal pumps.  The only benefit to putting flow restriction at the thermostat is to increase the coolant pressure in the engine.  If you are running at speeds greater than say, 6500 RPM it may (depending on other things) reduce the risk of pump cavitation.

It would seem there are only about two possibilities; air trapped in the pump housing, or incorrect flow path resulting in flow recirculating in the engine.  But then, you sems to have worked those.

Al