The task killer is probably using as much power as it is saving... in modern operating systems (like Android) having a program in memory uses no more power than empty memory, so the only thing being saved is the processing power for any background tasks being performed, which are pretty minimal for most apps... and the trade-off is you now have a task killer that is constantly looking for tasks to kill.
One thing to consider... the radio(s). Turn off any radios you are not using (GPS, Bluetooth, wi-fi, NFC or 3g/4g data) On a different phone, I shut off all data connections, and suddenly could get a couple of days off a charge instead of 1 day. So, I started using airplane mode extensively on that phone and got great battery life, and only a short delay when I needed online access. On my Optimus G (Sprint), I use Juice Defender to turn off the radios when the screen is blank except when it turns them on periodically to allow notifications and updates. Something like Juice Defender is good as long as you don't need instant email notifications etc. If you can wait 15 minutes for data notifications, you can save your battery with an app like this. Additionally, if there are things you need occasionally, but can usually do without, try finding a good widget set to toggle those radios more easily when you need to swap them instead of manipulating via the menus.
Another thing to consider. I assume the AT&T version is able to manipulate settings with the NFC tags like the Sprint version, you may want to use that capability to manipulate settings depending on where you are... the down side is that then you must keep NFC turned on for the few times when you need to switch modes... I suspect that NFC is pretty low power, so it is worth considering, I just haven't tried it yet.
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