Personally I think the existence of the extra-galactic synths is the least interesting thing they could have done to wrap that season up, followed only by Picard dying and instantly coming back to life before any characters even have time to process his death.
Oh, wait....
The theory I saw floating around that I really liked while season one was still airing was that it would have been revealed that the Romulans were actually organic synthetics created by the Vulcans, who rebelled and escaped, and then erased the memory of that fact, with only the Zhat Vash aware of the truth, and working to make sure no one else learned it, because of what that could potentially mean for Romulan society.
However, Michael Chabon and Akiva Goldsman didn't call me up for my opinion before the fact, and we're stuck with what we've got. So, no, I don't think the extra-galactic synths are particularly interesting, but they could be. However, at this point I'm certain we're never going to see any follow-up on them, except for in some novel someone eventually writes, which will have some elements almost immediately contradicted by canon material.
I learned it from a comic book.