As I recall, the Z-series is built by Sirtec (or Sirfa, can't keep those two sister companies straight) which has an even less impressive reputation than Impervio, which produced the infamously short-lived OCZ EliteXStreams.
GameXStreams are apparently made by Fortron Source, which has been coasting on residual reputation for years.
I know both lines fairly well: had a 600W GameXStream fan go on me, was replaced by two 800W EliteXStreams (both died), then an 850W GameXStream (arrived with a bad fan), and finally ended up with a 900W PC Power & Cooling Silencer which is anything but. Eventually yanked the stock 80mm fan and swapped it to a slower one since my system never even hits 50% capacity.
I was offered a Z series, but took the Silencer because I couldn't find a single review of the Z series two months after launch and didn't trust OCZ's ability to pick manufacturers. Reviews of the Z later popped up on a few sites and the results were about what I expected -- questionable performance, and every review found at least one meaningful defect. Not all the same, so it was a QC issue rather than just design flaws. E.g.
OCZ Z Series 850W Review