Hi Jack, yes, the yellow sapote from you tastes very good. I also got to try the Clytia fruits and I think that was even better. I have one more Clytia fruit (small one) I picked and will try it soon.
So far, the C. tetrameria #6 is growing slowly and I have not had any fruits on those grafts yet.
I do agree with you about the rootstock and using the C. edulis rootstock and grafting with C. tetrameria. I have some seedling rootstocks (from CSUF arboretum fruits) of both C. edulis and C. tetrameria. The large seedling tree I had growing in my son's yard was the McDill seedling, that tree grew over 20 ft and trunk grew large (5-8" diameter) in 7 years. On that tree, I grafted most of my 34 white sapote varieties except the C. tetrameria.
At my yard, I had the same seedlings from CSUF of the McDill and Yellow Sapote, but these were grafted earlier (3-4 yr old seedlings) with a combination of C. tetrameria and C. edulis varieties (24 total). All four of these trees have never grown taller than 10 feet, and the trunk size has stayed smaller (2", 2",3", & 4" diameter). So, it does make sense to graft them at an early age with the Casimiroa tetrameria to keep the tree smaller.