Running without a thermostat ok?
I personally believe it was aeration caused by the restrictor plate, which resulted in less surface are in the coolant across the radiator
I doubt this is the case. It could DEFINITELY aerate, but by the time the coolant gets through the upper hose it should have subsided enough. If it really was doing this, if you ever popped the cap with it running (warm or not) you'd have had lots of froth in the upper tank of the radiator.
There's lots of things it COULD be, but is shooting in the dark without an impeller map (like a turbo map, basically) to see where the impeller would break down at higher back pressures. Then you'd basically need a coolant 'boost gauge' to see if your impeller was operating in that island or not. Lots of info that probably isn't readily available and would vary significantly depending on the type of coolant, it's temperature, age, etc.
The other way, which is probably how the OEMs did it back in the day, would simply be to see how much pressure builds up in a particular engine configuration based on RPM and then target maximum pressure to coincide with max power production RPM. The electric pump could be 'qualified by similarity' if compared side-by-side with an OEM pump and comparing the pressure behind the tstat. The electric would have to at least match the OEM max at the OEMs 'sweet spot' to be worth it.
A 'step down' in pump performance may not be much issue in a mild race application since you can still pump coolant in between runs with an electric pump - more difficult to do that with a stocker.
