First off, I don't think water pumps on model A's are pumping too fast.
Next, it appears regulating, (slowing the flow of coolant) has solved or masked another issue. (my point is the car worked fine off the assembly line)
When I had this overflow problem and did a flow test (there are blogs on this site on how to accomplish) on the radiator, I discovered restricted flow. And I found the baffle at the top hose neck was gone! I tried lowering the coolant level, and just no help. I took the radiator to a shop in Missouri, and they gave me a price for fixing it right! I chose to buy a better radiator. Once replaced with coolant levels about a inch above the core, no coolant out of the overflow and all is well. Let me just say, the thermostat method used sound plausible and that would have been a cheaper fix if it would have worked on my restricted flow original radiator. In my view, if it works, all is well.