The beta program is only open for the single-SIM variation of the Xperia X Performance, which has the model number F8131. The first Nougat firmware is build 39.2.A.0.248, and it contains the October 1 security patches.
Split-screen multitasking is in, as is the new notification panel. Sony has also added new options for the camera app, with manual shutter speed control and manual ISO control at 23MP. The update comes with the latest iteration of Stamina Mode too. Touch sensitivity has reportedly been improved, and the auto-rotation bug is now fixed. Otherwise, not a lot seems to be changed.
Expect Sony to keep refining its Android 7.0 software, perhaps releasing further beta builds for the X Performance before the final update hits the streets.