I didn't read the entire thread, but I've personally had a "high flow" thermostat cause the coolant to not stay in the radiator long enough to exchange heat with the air. It's nearly the same issue as no thermostat, minimal restriction.

Hold on...doesn't the coolant have to have more time in the radiator to cool?
No. But a lot of people still think so. We have come up with some explanations for the Doubting Thomas.

Debunking the I Can Have It Both Ways Theory

The water has to have time to cool argument is most common one we hear. In a closed loop system if you keep the fluid in the heat exchanger you are simultaneously keeping it in the block longer. Unfortunately, the block is the part that is generating the heat. Sending hot coolant from your source (engine) through the heat exchanger (radiator) to the sink (air) will transfer heat as long as there is a temperature difference between the source and sink. The engine is still generating heat the whole time so why keep the coolant there any longer than you have to.

Debunking The Conscientious Electron Theory

We hear that the coolant has to stay in the system longer to cool but what is heat transfer really but conduction, convection and radiation of electrons. The fluid in your system transfers those electrons based principally on the source-sink differential and the exchange material's transfer rate. An electron moves at varying speeds - Bohr's model has it moving at 2 million meter/second. But let's just agree it is fast (really really fast). Far faster than the flow rate of the water pump. Your engine coolant's electrons do not know (or care) how fast you send then through the system - they just knows that the source is hotter than the sink and off they go.

Debunking Grandpa's Flathead Washer Theory

"But wait a minute, I know Grandpa' used to put washers in his flathead to slow the flow and cool his engine." We know people did this too. They still do it but the cooling benefit is not from the slower flow but the pressure that builds from the restriction. Consider that Grandpa had two flathead water pumps sending twice the volume through the same size radiator core. In a non pressure system he likely lost fluid on the track or road. We have use pressure caps since the late thirties to remedy this.

Ask Grandpa and he might tell you his overheating woes came when he tore up the track at high speed. The overheating could be the result of cavitation in his pump due to the higher rpm.

Restricting his flow with a washer build up his pump pressure and the pressure in the block helped reduce the onset of hot spots on his cylinder walls and formation of steam pockets. So Grandpa was on to something but just not for the reason most people think. This restrictions makes sense when your rpm is excessive but it rarely makes sense normal driving conditions.

If you doubt this thinking then try this simple Ask Dr. Science experiment where you restrict the pump on the suction side; just clamp off the lower hose while you watch your temp gauge. Hopefully, you will debunk Grandpa's theory yourself before you experience vapor lock.

Restriction is not all bad if it serves to prevent cavitation. Cavitation occurs when a pump turns so fast that you generate lower pressure and air bubbles or vapor forms. These bubbles eventually implode and damage the engine block wall and impeller. Rapidly spinning the impeller can literally rip the air from water but may not actually move the fluid, it's tantamount to turning an eggbeater in a paint bucket. Restricting the fluid flow to raise system pressure in the block may help prevent cavitation at higher RPM but is it necessary for most vehicles?

No. Most vehicles do not need to restrict flow because they do not reach or sustain high RPM. Additionally, thin aluminum radiators already restrict by design e.g. fewer rows of tubes. Restrict it further and you may as well hose clamp the lower radiator hose and we know how that works out. When you face Grandpa on the track you may want your washers, otherwise, keep them in the toolkit.

Simply put, you have a far better chance of keeping your cool with a greater flow rate through your heat exchanger than gathering heat in your engine block.