Why not do the test on a car using an infrared probe. They aren't expensive and it will save taking off hoses etc. etc.
The cheap ones may not be especially accurate but accurate enough to see different opening, running and closing temperatures. See here:-
Possibly also accurate enough to give a good indication of the accuracy of FES/ECU/Sensor temperature reading but do remember FES is dependant on ECU and ECU is dependant on the sensor (hip bone is connected to....) so compare a few different cars before coming to a conclusion.
Personally I think that the difference in 8 degree opening temp is a bit irrelevant to worry about and cause this much frustration. This, IMHO, is quite small when warming an engine and no more than a few minutes (probably less) once driving. Surely the average running temperature and a thermostats ability to react quickly and accuratly without being under or over damped is far more worthy of the frustration in this thread.
When driving my son's 147 I have seen the temperature (via FES) swing from 92 - 98 - 84 in a matter of seconds. Just drive quickly, stop at a roundabout and then get up to speed on a cold day - it can be done in less than 20 seconds. You drive then stop, the heat quickly builds, the stat opens and starts to cool the car, you drive away and the rad gets a blast of cold air and the stat takes a few seconds to react.
What does this mean to the engine? Provided it gets to temp quite quickly then surely oil temperature is far more worthy of debate or have I completely missed the point.
PS: these probes are very good for fault finding a misfire, uneven runing and checking carb balancing. Just point it at each exhaust branch in turn and you can see which one is running cooler. Lots of other uses too.