It's installing right now. The website from Sony doesn't say anything about this update. It describes an older one. This is strange.
I'll keep this post updated.
Update 08:36: Okay, so it's Build 23.4.A.1.232. I could have sworn it showed me a different number on the upgrade screen when downloading. The Website states the newest update is .200 from 2015-09-23 which is not this version. Looks like a bugfix update.
But, Google Photos stopped working. It crashes at start, the app can't be invoked at all. I had to go to the app settings, clear all data (you need to set it up again after that), and restart the app. It works again.
I'll keep you informed here if I know what's up with that update.
Update 2015-10-10 16:22: Yup, like expected it's a bug fix. It removes the Stagefright vulnerability (oooh so scared...not). No other changes for sure. I heard about changes in battery consumption and display brightness changes on some models. But you know some people see all kind of stuff after an update ;). However with >200MB for this update, there is likely something other. I hate when companies don't publish a proper changelog.