The aspect that some forget is this:
Until the thermostat opens, the coolant in the engine near the thermostat isnt moving. Convection (hot rising, cool falling) means that the coolant right around the thermostat must pick up to AT LEAST (and then well past) the thermostat opening point before it reacts by opening. By the time it opens, the coolant is over that rated temperature - meaning it has to play "catch up". For proof - next time you jump in the car with the engine cold and then get it up to operating temperature, you'll see the thermostat open (late, higher than its rated temp), then the coolant temp falls down below its rated temp, then it swings back higher again and finally settles on its happy place. It doesnt seem to matter whether its a standard stant or Robertshaw type, they all seem to do it.
Drilling a hole in it does marginally lower the temperature rating, but it allows coolant to flow past the thermostat and allow it to react to cold-start warmups without the temperature swings. I've never drilled one and not been happy with the way it operated. I can't say the same for ones straight out of the box.
Older thermostats (ones that were made up into the 80s) had a small passange made into them for this purpose, but its not there anymore from my checking.
My opinion is that thermostats are trying to establish a miminum operating temp for the engine rather than trying to regulate a maximum temperature.