Damn, really? I've read so many conspiracies and articles about the Zodiac Killer being "discovered" before that even if this one is true, the conceptual Servant "Zodiac" would probably be stronger than this Gary Poste guy.
Not to mention Nasuverse's general rule of more mystery --> more powerful.
On a related note, I saw this Reddit post once on a Zodiac Killer fanservant as a Ryuunosuke pseudo, which I enjoyed simply due to how wack it is.