I was speaking with Bill Mee yesterday about Sweet Cliff lychees. He related that Bob Murray, former owner of Treehouse Nursery in Bokeelia, told him that he was unable to graft lychee to a longan rootstock, unless it was a Sweet Cliff lychee scion. This is the only one that would take. He felt this supported the idea that Sweet Cliff is a cross between a longan and lychee. Besides, to me, it makes sense, when you consider the texture (more firm) of the flesh, the slightly musky aftertaste, the size of the fruit, and the tougher/smoother skin.
Brad