Thread maybe belongs in "Cases/Cooling," but no complaints.
I'm just surprised that this topic keeps bubbling up over the last few years.
It all boils (ha! pun!) down to thermal resistance.
I think it was almost five years ago -- six months after the release of IC Diamond -- that I actually ran tests at controlled room-ambient to compare AS5 and IC Diamond.
You get so many degrees improvement using the diamond paste -- I think I was able to prove something between 2C and 5C. It depends on the thermal wattage being generated under load conditions . . . so it's "something" more . . . or less . . .
You get again 1 or 2C improvement over IC Diamond for using the metal products like Indigo Xtreme, Coollaboratory pads -- probably the liquid metal pro.
If your IHS is nickel-plated (and it is . . . ) and your heatsink base is nickel-plated, you can probably get up to a 5C improvement for each surface lapped . . . . which becomes another issue if you worry about warranties and resales.
And again -- these numbers in degrees C depend on how much thermal wattage your processor puts out at your favored over-clock setting. I think my numbers were based on a 95W TDP processor that was over-clocked to generate something between 115 and 120W . . . .