Thermostat operation is critical on modern engines. The fuel injection bases its fuel map off of engine temperature. Personally, I always use OE thermostats as a result.
Also, thermostats do not create pressure. That is the job of the radiator cap, which also helps maintain proper temperature. Radiator caps are also important to the cooling system operation. Much of this was covered in the Engine Overheat article I linked for you. I believe it also has this video embedded.
Please continue to keep us posted.