Perusing the inter web, came across this.......

It could explain the “gets hot without a stat” side of it:

“Removing a thermostat from the cooling system can cause higher engine temperatures due to a not so obvious behavior. Gases are a poor conductor of heat. For there to be low thermal resistance between the cylinder head coolant passages and the coolant, the coolant must stay in a liquid state. If the coolant changes to a gaseous state, the thermal resistance goes up massively.
If you model the pressure drops around the cooling system in a loop, using the fluid dynamics equivalent of Kirchhoff’s Voltage Law, you will find that removal of the thermostat results in a much lower average pressure in the cylinder head coolant passages. This allows the coolant in the cylinder head to boil. Once the coolant is in a gaseous state, very little heat transfer occurs.
This is why some racers install flow restrictors in place of the thermostat. It creates the required pressure drop, to keep the fluid behind it at a high pressure. I learned about this requirement from an OEM engineer who measures engine behavior with 20+ coolant temp and pressure sensors installed throughout the cooling system. In 30 years of automotive work, that was the only person I’ve ever run across that could explain this behavior.”

Stewart recommends no restrictors, but they also recommend the highest pressure cap the radiator is rated for.
That might be an interesting test for the OP. Find out from Griffin what pressure the rad is rated for and run a cap real close to that, either without a thermostat, or a fairly large restrictor, and see if it has any affect of your findings.