It should be noted, too, that compounds are tailored to package type. Ceramique works really well on exposed silicon - chipsets, PLX bridge chips and so on, but not so well on the larger areas of heatspreaders. If mayonnaise is OK as a compound, the difference between top end (and expensive) compounds isn't likely to be much. Certainly application technique, including surface preparation, would be more important than the compound, and possibly more useful for inexperienced people to read than parochial bickering.
I must admit, finding out whether diamond dust will scratch a mirror is a curiousity though. I know that MX2 will very slightly abrade copper - it removes enough metal to leave a deposit on a plastic application sheet, but copper is so much softer than glass that's not much of a comparison.
