Finally Android 4.2.2 Jelly Bean update is rolling out for Xperia Tablet Z (SGP321) users but
only for the owners of the LTE version of the tablet. The build number is (10.3.1.A.0.244) i.e same as that of available in Sony Xperia Z, Xperia ZL and Xperia ZR.
Its also said that the Wi-Fi version didn’t hit yet, but it likely seems to be hit at some point after the LTE version. As you get the update tell us via comment below.