I wouldn't consider them synonymous.
Upbeat, other than the strictly musical definition of the last beat of the bar, is an expression of positive sentiment & is not exclusively used to describe music. e.g. this definition & these synonyms.
Up-tempo music, on the other hand, describes a fast tempo. It may also imply positive mood (can't find the example i saw of this implication the i saw this week, damn!), but generally not. e.g. this definition
Now, as for your identity relation: i don't think that hold either. According to a quick bit of research that i did:
downbeat refer to a style of ambient music;
downbeat is the first beat of the bar (as you already observed), & by analogy it also applies to the beginning of a rehearsal or performance;
downbeat is the opposite of
upbeat in the sense of meaning a negative mood as opposed to the positive upbeat mood.
downbeat are opposites in terms of mood (positive/negative) & complements in terms of positions in the bar (last/first),
downtempo don't seem to have a similar relationship. Further, while the words
downtempo refer to a style of ambient electronica, they are not interchangeable in any other use.
uptempo are not synonyms;
downtempo are both names for a style of music;
upbeat mood is the opposite of
downtempo aren't really antonyms as the former is a general description & the latter is a specific style.