Hehe... The comparable Meguiar's products would be either #82 or #9, the "Swirl Free Polishes". #81 and #7 are similar products to each other but not to SFX-3, both adding gloss producing oils but offering nothing in the way of true "polishing" (levelling paint). #7 is preferred for single stage paints, #81 is preferred for clearcoats, but either one will work on either surface.
The confusion comes because there's no standard meaning for the words "compound", "polish", "glaze", "sealant", and such. Believe me, you're not the first one to scratch his head at this conundrum.
I really like the SFX-3 AND the #82. Based on my experience, the #82 will work on somewhat more serious marring, the SFX-3 removes less paint and would be preferable on minor marring. Either will do the job when used properly. Neither is really powerful, but #82 is about 1.5x as strong, IMO. SFX-3 is like jeweler's rouge; it's "final finish polish" in the most real sense. Use it when you think you're satisfied with the way your finish looks, and it will take it up another notch.
I have no experience with the Z-PC, can't answer there.
Tom