Just saw your post. I have only flavor supreme and flavor king grafted on a peach tree (added small Santa Rosa branches this year and had a Santa Rosa plum tree 50 yards away) and flavor king has absolutely loaded like grapes. Flavor supreme also fruits well but not as crazy as FK. Just FYI.
Btw, I am in mid Atlantic area. It’s said FS was difficult to fruit for many people in east coast but not my experience.
Another thing: I bought the Santa Rosa last year from Lowe’s because I was told that FS and FK had difficulty pollinating each other. But my 1 year branches of FS and FK fruited profusely last year. I also grafted FS and FK on Santa Rosa last year and this year FK fruits like grapes and FS also fruits very well on that tree. After thinning I have about 20 fruits on that small tree including 4 Santa Rosa.
So the mystery of plums/pluots pollination remains mystery at least for me, but in a good way.
Your FS has excellent set! Where in the mid Atlantic are you located? I used to live in Wilmington DE, and it didn’t set well for me there.
@JamesN how does your climate compare to the Inland Empire Riverside/Redlands/San Bernardino? Your photos show a lot of green around you, whereas our habitat, if you want to call it that, is mostly brown in summer. In July our high has reached 90-100, cool at night 60-70. I think we get 300-400 chill hours in winter.
Did we meet before? 
Sorry about the disappointment…but at least the mystery was solved. Dapple Dandy can be excellent if you let it get soft ripe, with blackish red flesh color. Here in CT it reaches that stage around mid October.