Since the Cores are calibrated to preserve mean TJ Max 100c, these results should be valid.
Intel does not adequately document TJMax, even in their new Core i7/i5 processors where this value is written into each core. Sure, most Core i7-900 series processors have 100C written into each core but no one really knows what that means. There's no documentation to show how much error is in this value and no documentation to say whether TJMax = 100C represents a minimum, maximum or average value. Your guess is no better or worse than my best guess because there isn't any publicly available documentation to back our opinions up.

I totally agree with your 5C findings that what SpeedFan reports as the CPU temperature as read from the thermal diode should be 5C lower than the core temperature. The problem is that whatever this thermal diode temperature reads, no matter how accurately you are able to calibrate it, that is still not a measurement of the Tcase temperature. You can't compare a thermal diode reading to the Intel Tcase specification because these are based on two different measurements at two different spots on the CPU.

On my motherboard, I can't calibrate this sensor using SpeedFan because as the temperature increases, the amount of error in this sensor also seems to increase. At TJMax when the core temperature is being reported at 98C, the CPU sensor is reporting 107C. Based on your 5C results, it should be reporting about 93C which is a 14 degree error. At a lower temperature where the slope error of the core sensors is still minimal, it reports 72C when the core temperature reports 67C. Based on your 5C findings, the CPU sensor should be reporting 62C so now the error has decreased to 10 degrees. At idle, the amount of error in this sensor has decreased again and now the error is somewhere around 6C.

I don't know if this shows a problem with this sensor or more likely just the calibration correction factor that Asus decided to apply to it. Using a single point offset like SpeedFan allows you to do can make this reading more accurate but no matter where you choose to calibrate it, the readings at other points along the temperature curve will either be too high or too low and not 100% accurate.

That leaves me with a sensor that can't be properly calibrated that displays a number which Intel has published no specification to show whether that number is good or bad. I can now see that if your core sensors were damaged or stuck then you might be able to use data from this sensor to approximate your core temperature as long as your motherboard has applied an appropriate calibration correction to it. In reality though, almost all core sensors work perfectly fine in the 70C to 125C range where they were designed and calibrated to be reasonably accurate. The CPU diode temperature might be useful for users with stuck sensors but sticking sensors at normal temperatures isn't an issue anymore for the Core i7/i5 CPUs.

Intel saw the limitations of using the analog diode sensor which was their motivation to introduce core temperature sensors. It makes a lot more sense to thermal manage a CPU based on information coming from well calibrated sensors mounted on the hottest spots on the core. Even 45nm Core 2 sensors that have a list of issues as long as your arm still work excellent in this upper range. TJMax is not properly documented so using them to report an accurate absolute temperature is a crap shoot at best. They are perfectly fine though to compare your core temperatures from one day to the next and to determine if changing your heat sink or fan or thermal paste improved your full load temperatures or not as long as they are not sticking.

My findings are the same as what rge has found. Core temperature isn't that important until you are overclocking your CPU to the very limit of what it is capable of. At that point, for stability, lowering your core temperature as much as possible becomes very important.

I've spent a pile of time too investigating CPU temperatures and my conclusion is simple. Run your Intel CPU as cool as possible and you will increase your stability while maximizing the amount you can overclock it. As long as your computer is stable and not setting off the PROCHOT# bit and thermal throttling, then you can pretty much ignore your CPU or core temperature. I hate to say it but CPU temperatures just aren't that important anymore. Intel makes some great CPUs that do an excellent job of looking after themselves, even when pushed to the outer limits so there's really no point in users splitting hairs trying to come up with 100% accurate idle temperatures. It's been a nice hobby and I've learned a lot but it's time to move on.