Have root access on 15.4.A.1.9 (C1905 locked bootloader) and everything seems to be working on par or better than 15.4.A.0.23
So unless you know of a specific feature, not functioning in 15.4.A.1.9, then I wouldn't hold back on upgrading.
I note that OpenGL ES 2.0 is still giving benchmark results well below that obtained on 15.1.C.2.8 (Android 4.1)
Guess that's the price we pay for OpenGL ES 3.0
Sony notes the following in the Xperia M software update page:
Android version: Google Android 4.3 (Jelly Bean)
Latest available software: 15.4.A.1.9
Release started: 2014-06-10
Included in this software
This upgrade for your Xperia™ smartphone includes many improvements, such as:
Enhanced user experience
Improved battery life
Software performance improvements
Not really a change log, more like an update overview.
It can always be placebo effect, but I feel the GUI is more responsive.
Battery life is always subjective depending on usage.
On Android 4.3, I have not had any
reboots outside those I have manually initiated.
I have 'gutted' the /system/app folder of over 26 apk.
Did have Spiderman theme loaded for a few minutes, just to see if Xperia themes worked and then deleted it.
I did a manual reboot after loading the theme.
Currently running Malaysian 1274-5466 build.
I built it from SUS download via Flashtool and extracted system.ext4 for root access upgrade.
On 15.4.A.0.23 I was running a Serbian 1274-5505 build.
I did note on the Serbian 15.4.A.0.23 build it had SMS character optimization.
This is something that is missing in the current 15.4.A.1.9 Malaysian build
Just something to consider for those using extended character sets in SMS messages.