Sony Mobile is now pushing the latest firmware update for the Xperia XZ Premium and XZ1 family, moving the build number from 47.1.A.12.270 to version 47.1.A.16.20. This latest update includes the 1 September 2018 Android security patches. So no Android Pie just yet, but it has been caught testing on the Xperia XZ Premium, so hopefully Sony will meet its November deadline.
Thanks Blasse, Dennis, mortael, mrninko, Robert!