The t'stat performs two functions:
[1] It maintains a minimum operating temp for the engine.
[2] It creates pressure in the coolant in the block to suppress nucleate boiling: coolant boiling in hot spots like near exh valves, where boiling creates air bubbles. Air is a very poor conductor of heat, so any bubbles should be reduced or preferably eliminated. That is what the stat does.
If you measure all the coolant hole areas of the head gaskets, you will find the combined area greatly exceeds the flow area of the stat. The stat area is deliberately smaller to create the pressure point. If you do not use a stat, you should use a washer of the same area.