Were I a Federation judge, I wouldn't classify him as legally Human; I don't think Federation law should have any such status as "legally this species" or "legally that species." It makes more sense, I think, for Data to be legally considered a United Earth citizen (as Omicron Theta was a U.E. colony) and a Federation citizen, and to be considered their legal son.
Good point. "Legally" was a poor choice of words. Still, it never occurred to me before to think of Data as essentially a posthuman intelligence, but it fits. The term is often used in SF and futurism to refer to AIs that are downloaded or simulated human minds or that were created by humans as our successors/replacements (as distinct from transhumans, which I think of as referring to genetically or cybernetically augmented humans, though the terms are often used in overlapping ways). Data was designed in his creator's physical image, and Soong's purpose was presumably to make an android that replicated humanity as perfectly as possible (judging from how deep Data's desire to be human is). And he was originally conceived as having the memories of the Omicron Theta colonists within him, though that was later retconned to just their journals and personal files.