Well, I've been keeping a close eye on my temps lately, in anticipation of a comparison with you...
My socket varies from 5 to 8° lower than the diode temp.
I can't explain the variance-it doesn't seem to correlate with a particular activity and my diode stays fairly constant through most of the day.
Folding or not, sidepanel on or not...my diode stays between 32-35°C.
This is ignoring the temps I see first thing in the morning before the heat kicks on.
Typical ambient would be in the low 20s, I would guess.
If I remember correctly, the temp differential was more on the order of 3-5° when I was using a 80mm fan.
Just extrapolating, I would guess that you might see socket temps maybe 2-4° lower than the diode after you are up and running.
Why must you strip everything out to switch from the Zalman to the Thermalright? Is you mobowall hole not large enough to access the mounting screws?