Emitters are tested and sorted by performance, so a Z-bin (if it existed, which I doubt) would be extremely rare, very bright, and really expensive. Some small number of people have Luxeon V lights with an X-bin emitter, also known as "unobtainium" because of how hard those things are to obtain.
The Cree XR-E has bins as follows, from worst to best (in terms of output at a set drive level): P2, P3, P4, Q1, Q2, and I'm pretty sure that's all that's really available right now.
With the Seoul P4 emitter (also sometimes called SSC P4), you'll usually see lights with a U-bin emitter. That would be better than a T-bin emitter, but not as good as a V-bin emitter.
I hope some of that helped.
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