Based on some expert advise I got regarding the same subject, following is what i understood.
Taking off the Thermostat would make the engine take longer to generate the optimum operating temp thus having serious repercussions on the life of the engine plus lower mpg's..
Although the removal of the thermostat would make the vehicle cool better on low speeds( rather work at a lower temp), on higher speeds it could lead to overheating for the fact that the water circulation would be too fast to absorb the heat efficiently. (I'm assuming this is due to the water pump driven faster..). A thermostat would open up depending on the temp and regulates the flow of water, so without it its just free flow.
On the issue of the Nissan Genuine Thermostat giving trouble, could it be that the opening threshold temp in the selected thermostat is wrong? I saw the opening temp engraved on the wide wall of my vehicle the last time the mechanic took it out..
Correct me if I'm wrong guys..