Yesterday I took an extensive road trip where I knew I would both be unable to use the phone (while driving) and be nowhere near a charger (although I have a spare external battery for real emergencies). I decided to throw the device into extreme powersave mode. Did not discern a significant advantage over "regular" power saver mode, but at nearly 17 hours off charger (with some calls and texting, a little wifi out of the mode and a little data) I was down to about 25%. That works for me.
Beamed from the holodeck of my Nexus 7 FHD.