Maybe I had a bad one, but the drilled thermostat I put in my Silverado kept it from warming up as fast in the winter.
Either way, its true intention is to allow a small amount of coolant through to keep the heads cooler instead of a stand still of coolant which can heat soak them. That way, when the thermostat does open, the heads are ready to go instead of being extremely heat soaked and in need of cooling.
The holes are not large enough to make a big difference if it sticks shut.
From Wizbang at EasyPerformance:
The 180ºF thermostat was modified by drilling four small hoses around the thermostat body to permit a small amount of coolant flow through the cooling system even when the thermostat is closed. This modification results in reducing the thermostat's opening reaction time when the engine is relatively cool to begin with and is then challenged with prolonged wide open throttle runs. Without a drilled thermostat, the temperature of the cylinder heads can become quite elevated before the thermostat has a chance to open and establish coolant flow through the heads to cool them down.
