The prerelease one that someone here has. I thought it was you?
No, not me. Mine gives 4X @ 5GT/s in slots 3/4 but it suffers from exhaustingly slow boot and disappears from the Option-Key boot picker if using an NVRAM reset. Also seems to produce or facilitate other strange anomalies regarding BT connections and other strange things.
In Slot 2 it gives me 4X @ 2.5GT/s but ALL other problems melt away. Boots crazy fast, BT has never worked better, and other small issues are gone. So, it stays in slot 2 for all around performance and ease of use, with the loss of 1/2 transfer speed (which is only noticed with benchmark utilities).
If someone has 4X @ 5.0GT/s in an SSUBX 512GB blade installed into Slot 2, it surly isn't me. That's exactly what I would like to have, but I'm happy with the current performance. It would be nice to get the full potential out of this thing though.