[JayChicago]

Something doesn't make sense here. The big improvement was noticed at low speed in traffic. Pumps aren't pushing much water then, seems the restrictors shouldn't make a difference. One thing not mentioned in the original post so worth bringing up: if you fill the radiator to the top leaving no room for expansion, it will overflow when hot, usually occurs in slow traffic.

Also my two cents on recovery tanks:
I don't get the logic of a recovery tank. I don't think there is any purpose to it.

Modern cars use a remote recovery tank because their radiators use the entire space available for the fin tube heat exchanger. Then a remote expansion/recovery tank is necessary.

Our tall radiators have an expansion tank built-in to the radiator, the couple inches above the heat exchanger. Having a second remote expansion/recovery tank serves no purpose. It will allow you to fill the radiator to the top, but that extra quart of water filling the expansion tank above the heat exchanger doesn't do anything to help the cooling system. After the engine is up to temperature that extra water is just sitting in the remote recovery tank, where it is not doing anything.