Interesting conversation!
For what it's worth, I've known Gary from TimberTools for many years and he knows - through long experience - both belgian stones and eschers. On his site he differentiates between higher grade belgians and lower grade belgians. This is key, in my opinion, to understanding what he's saying about not following a high grade belgian with an escher. There is, I have found, a big difference between standard grade coticules and Select grade. I don't know if it's true about all of the newly named coticules from particular veins but there are differences between standard grade and select grade that go beyond cosmetics. I'd like to hear Gary weigh in on in this as, like me, he's handled hundreds of coticules.
I also have a number of eschers in my study collection ranging from the blue-green ones to the yellow-green. I don't think an escher following a Select coticule will get you anything measurable in the way of results on your face. The big difference, as I see it, is the difference between quartz crystals and garnet crystals. The structure at the micro level shows up as differences at the macro level. Tiny cleaved corners off of a rhombic dodecahedral garnet (nothing cleaves like a garnet!) in a slurry will give a smoother cutting edge than hexagonal, long c-axis quartz crystals. That's what I've found.
The differences we're talking about here are probably overshadowed though by factors like differences in the wiriness of your beard and shave preparation. A quality escher and a quality coticule are excellent finishers when used correctly.
To make matters a bit more complicated, I find that Shaptons in the 16k to 30k range will finish a razor off even better! I know that may sound like heresy to some but let me assure you I have not lost my love of natural stones and still use a combo coticule on my razor and it's still in my medicine cabinet and used on my razor every week. However, when playing with a new razor, I rehab with the Shaptons because it's quick, organized, and gives me consistent and reliable results every time.