Up to the point of cavitation, more flow removes more heat. The reason thermostats open with heat is to increase flow.
There's pretty-much no such thing as "flowing too fast to pick-up heat"; or "flowing too fast through the radiator to remove heat".
He explains why removing a thermostat will change the flow through the radiator and cause an overheating issue, but this is all based on an inefficient radiator.
Removing a thermostat "can" cause overheating; but the common reason is that the vehicle has a downflow radiator. The radiator cap is on the hot side of the rad core, which means it's also on the higher-pressure side if the core has any restriction (and it always has some restriction)
So you remove the restriction caused by the thermostat, and now the rad cap has system pressure PLUS water-pump pressure...and at high rpm, (high water-pump pressure) it blows coolant out the overflow until there's not enough left in the cooling system to keep the engine at proper temp.
This is so much less likely on a cross-flow radiator, since the rad cap is on the cool tank--the
suction side of the water pump.
He also states that most factory cooling systems are not efficient enough.
Efficient enough? Or "big" enough?
Trucks and heavy equipment tend to have H-U-G-E radiators compared to passenger cars and light trucks. Reason being, those engines are worked at near full-capacity for hours at a time. Passenger cars/light trucks are generally run at part-throttle--lower power level, lower average heat load. When the engine is run hard, the temperature rises, and we're used to that. We think it's normal unless the temp rise is too extreme.
Exactly.