A bug-fixing and performance update is heading to the Xperia SP (C5302, C5303, C5306), judging by the latest PTCRB certification. Firmware build 12.1.A.1.192 has popped up on the site, which would replace the current update (12.1.A.0.266). This would move the build number from 12.1.A.0.XXX to 12.1.A.1.XXX.
Users have reported on a number of bugs within the Android 4.3 update, especially with the illumination bar (for example it remains on whilst on a call, some are also suffering from problems in the Album and WALKMAN apps). We’ll let you know when/if the update starts rolling.
Thanks Kamen and @Roden95!