I really disagree. I don't believe he had a good chance at being a Sneap-like figure. Look at the albums with Z as the primary mixer. They are very few. He did not mix CW, AoB or Resurrection, though he did co-mix AoR with the same guy as he co-mixed ToS with; who did what and how much is unclear. That guy has worked on loads of records though since way back, so I take it he wasn't Z's junior. Both ToS and AoR sound good, very good even in the case of AoR, but not great and sound a bit dated even in 2005 - compare that with Sneap's work with Testament in 2008. He co-mixed Crucible (which sound was messy and underwhelming, but thankfully was remixed in 2010. He is much more of a producer than mixer - he had way more producer credits and co-credits than mixing credits, whereas Sneap is primarily a mixer and producer secondly. In an alternate universe where Z was better at mixing maybe he could have been a Sneap, but it is fairly obvious his primary competence lays in other aspects of the producer spectra. Halford has referenced him at being great to work with while recording vocals, in coaching him and finding the right Halford voice for different sections and songs.
This notion of his skill inside the studio is what I mean with the mythological thing surrounding him in Maiden circles that isn't entirely fair and accurate. He worked on many great albums, but generally as a part of a team effort, not necessarily primarily responsible for how the record ultimately sound. That is not how things have generally been with Sneap, at least since his career took off 25 years ago.