1) Yes. All thermostats of whatever make restrict the flow of coolant through the engine more than no thermostats do. The neck on the head of my 59A is 1½ " I.D. = 40 mm. Even the Ford original stat only had an opening of 29 mm and that's the biggest opening of any stat that will fit in that neck or hose. That 29 mm opening only passes half the flow that the neck will pass. So, why use a stat if every one is going to restrict the flow even when fully open? That brings us to
2) Why use stats at all? Here are all of the reasons that I know and have read:
a. It warms up the oil quicker to thin it out so it flows better in the pressure system and splashes better up under the pistons.
b. It boils off the moisture in the crankcase that will otherwise contaminate the oil and do other damage.
c. To get heat sooner in the heater in the winter.
d. It prevents the rapid expansion and contraction of the engine block and all engine parts caused by rapid fluctuation in engine temperature and the consequential damage caused by such rapid expansion and contraction.
e. And . . . the Number One reason to use thermostats: Because Mr. Ford said so (and every engine manufacturer since).
So, understanding all of the reasons to use thermostats why would anyone go without? Only two answers I can think of:
a. Their cooling system is so sick that the mere addition of the restriction of even the best of thermostats causes it to overheat. So, instead of diagnosing and solving the problem with something else in the cooling system they do the easiest thing - remove the thermostats.
b. Because they've bought a water pump from a manufacturer that tells them not to use a thermostat with it. Why would a water pump manufacturer say not to use a thermostat? Because the manufacturer promotes his pump as higher volume than other pumps but knows that such is only the case if the restriction of the thermostat is removed because his impeller was designed to move more water only when there is no restriction such as a thermostat whereas all other pumps are designed to move more water through the thermostat. Why would anyone buy such a pump? Because they don't know or don't believe these facts because someone is repeatedly promoting the pumps without disclosing these facts.
Nuff said.