Ok back again. I have been researching exactly what the EXIF temperature is all about and it seems it has nothing,
absolutely nothing to do with the CMOS sensor operating temperature. It is built into the DIGIC processor and reporting the temperature of the processor. The DIGIC processor gets hot during sensor readout and processing, and seems to be responsible for most of the heat generated inside the camera.
There is an interesting thread on this on Cloudy Nights located here:
http://www.cloudynights.com/topic/50...s-sensor-temp/
Scroll down to Spokeshave's post (#8) which explains what is going on inside the camera and the role of the DIGIC processor. Spokeshave seems to know this topic well.
Hopefully people can get away from quoting the EXIF temperature in relation to camera cooling functions - if it does cool off at all it would only be a side benefit of cooling and is not directly related to optimal image sensor noise reduction.
It is reasonable to conclude that only direct reading of the CMOS sensor or the cold finger adjacent to the CMOS sensor, can give an indication of CMOS sensor operating temperature.
An interesting aside comment from my reading would be that longer sub times would seem to mean less frequent readout and processing by the DIGIC processor so less heat buildup inside the camera body. Typically cooldown intervals that we program between subs are really cooldown periods for the DIGIC processor. So logically the longer the subs the cooler the camera should be compared to more numerous short subs that require more frequent readout and processing.