It's true, the thermostat body at least is required to restrict flow otherwise the cooling system doesn't work. I tried it in my racecar to develop the ideal restriction, and found the best thing is to leave the thermostat in place. What you want for good cooling is a swirling of fluid ( turbulent flow ) against the cylinder wall and again through the rad. If you let the fluid flow too fast then the flow straightens into laminar - meaning the cool fluid just shoots through the middle of the flow paths without touching the hot sides.
