FWIW, I have an 64 gig, AT&T iPhone 4S and have upgraded
to 6.1 and have no degredation in battery performance vs.
pre-update. Personally, I have never had battery issues even
back to my iPod Touch 3g with updates. Perhaps it is my update
methodology, which won't help now but may with future updates.
here it is:
1. Connect to iTunes via USB and when prompted
about the update download only.
2. Wait for the download to complete and then procede
with the sync/backup.
3. Disconnect the iPhone.
4. Wake it and close all open apps.
5. It may or may not help, but I do a reboot at this point.
With all apps closed, it could be flushing all temp data.
6. Plug in then let iTunes do its thing with the update.
7. Then eject the iPhone and do a full power down and
then restart.
I have followed this method for a long time and have had
no issues with updates causing any problems.