I have not seen any Worlds Best mulberry fruit or tree, but for me the Thai Dwarf seems to fruit more than the other types I grow. If you have the Worlds Best or have access to it, I wouldn't mind trading a few scions with you. I have the following on my trees; Black Pakistan, White Pakistan, Shangri-La, Thai Dwarf, Contorted mulberry.
I top worked one of my mulberry tree with Thai Dwarf this spring and had fruits on them already. Taste is ok, nothing exciting, the Black Pakistan or the Shangri-La tastes better to me.
The nice thing about it is the growth habit, leaf nodes are spaced closer together so it seems to be more bushy even though it grows fast and long.