Drilling a couple of holes in these goes way, way back. It's done for steam or airpocket venting. To allow some flow when the t-stat is closed and to prevent air bubbles from forming. There's a reason the "speed bleeders" keep letting a bit of air out every month.
A much longer line of input I heard once was so there is always water running to the radiator which helps break up any trapped air inside the t-stat housing. The way the system works it that while at startup and lower temps the water does move. It flows constantly through the engine and circulates around and around. Not until the temp rises to 195 does the t-stat begin to open up, and as it opens, it restricts bypass flow and opens radiator flow. At 220 degrees the radiator is receiving 100% water flow and the bypass recirculator is cut off completely. It is during the warmup stage that water flow to the radiator is critical. If the viper is driven hard while the engine is cold; hard suction from the pump and lower hose will cause a draw of air into the system because the thermostat is not open. Meaning, the back side of the upper hose is trying to **** water from the t-stat housing but it can't. Therefore it tries to pull from the overflow bottle. If your level is low there, it will **** air and ingest this into the cooling system. How do you solve? Drill a couple of small holes into your t-stat to allow fluid movement to the radiator at all times. This will eliminate the suction to the surge tank and keep the engine fully primed at all times with fluid and not air. The surge tank will still have its function as a reservoir for fluid expansion when you shut the engine down. When the thermostat opens completely and the draw of water is needed, it can still overcome the cap pressure and pull fluid vaccuum from the surge tank.
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