Hi,
Firmware 2.20.40.58 resolve many bugs and other issues, thus, worth upgrading asap.
Like you, I did not receive this upgrade notification OTA on my 1 day old ZE2 and thus, I manually install it by going to the link below, very simple step like how your clear system cache but you need to have a SD card.
http://www.asus.com/zentalk/forum.php?mod=viewthread&tid=26180&extra=page=1
i've upgraded my 551ml to .58.
Battery issue:
Before i upgraded, i noticed there was a battery life issue where certain apps, once opened, stay running in the background and keep the phone in an "awake" state (i.e. in settings>battery>view details)
even when the phone was locked and the screen was off. it didn't really bug me much though because the drain was still manageable. Some of these apps were Mangarock, Facebook, 9gag, Sgag, etc. in other words, once i've opened any of these apps, the "awake" bar will basically be solid from then on regardless of the phone being locked and screen shut off for hours.
To fully manage them keeping the phone awake, I had to manually remember which apps i opened, then go to their app info and force stop after i'm done using them. A simple exit (e.g. back>back>back>back etc., or selecting the explicit exit option in the app itself) never worked
This would cost my phone around 3% an hour, and my phone could not be charged by a normal wall charger (i.e. 5v 2A) at all; it would actually drain power even if it was plugged in overnight
After the upgrade ytd, i noticed that the battery screen is finally displaying the "awake" portion as "off" at the timings when i lock my phone. I had purposely opened and exited all the above apps somemore (without force stopping them)
all in all, good upgrade. many of you may find that the battery life issues you were facing will be fixed, i'm guessing.