I guess it's risky because you might bend the pins (if it's an AMD chip). But that's not really it, people have tried this over and over and the results have varied from absolutely no change to maybe a couple of degrees. Question is, are you really that thermally limited such that this is the last option you have ?
^^ This exactly. Is your system running hot and throttling? Or are you trying to lower temps a few degrees in hopes of increasing longevity? Only you can determine whether the time and effort is worth it.
Varied improvements (if at all) after lapping are almost certainly caused by differences in flatness from CPU to CPU. No two will be exactly same. Below is my Xeon W3680 as example. Heatspreader was far from flat. Starting with wet 400 grit paper on a mirror, high spots in shape of a capital letter "I" became immediately apparent and was obvious I would be there all day. So switched to 120 grit to speed things up. Finished with wet 2,000 grit. When you get into surfaces like this TIM particle size becomes more an issue. Are they small enough to drop into micro imperfections in the mirror-like surface? Or will they sit on top and prevent the metal surfaces from touching much as they could? Which is what we want in an ideal situation - direct metal to metal contact.
Now the question is ... did this improve my temps? Hard to say since small drop perceived was well within margin of error. Switching to Kryonaut already dropped temps a few degrees over the AS5, and several degrees lower than standard white paste, even before lapping.
As for diamonds. I don't think Kryonaut employs them otherwise it would be a big marketing point. More likely they use Aluminum Oxide (aka - alumina) like most other ceramic non-metallic or silicone based TIMs. Excellent thermal conductivity, inexpensive, and readily available since significant portion of Earth's crust (read: 'bauxite') is comprised of it. And by the way. Aluminum Oxide is a very common abrasive and probably what is used on the sandpaper you will be lapping with. So no wonder it scars CPU surfaces.