If you take a look at the Xperia SP support page now, Android 4.4 KitKat appears as being “under investigation”, so it’s no longer confirmed as coming in the future.
Needless to say, the Xperia SP is more than capable of running Android 4.4 KitKat, which is optimized to work even on lower-end devices. Even more, the SP hasn’t turned 1 year old yet, so it’s still a relatively new handset. The bottom line is that Sony has no reasons not to updated it to KitKat. So let's hope that an update will be available in the near future.
source: Sony via Xperia Blog