2011 Prius Hybrid battery drain

what is considered bad for you? the hybrid battery should be fine until you get on the screen "check hybrid system". And then you got a P0A80 code, that is indications you need to work on your battery. Or you can do some preventive reconditioning with a system like the Prolong.
what do you consider a bad MPG? I got in my 2 prius really bad MPG on summer mainly because the air conditioning working hard, its electric so it discharges the hybrid battery quickly and the engine need to compensate that by trying to keep charged the battery which at the end affects your MPG. So having a new battery will not solve the MPG on this case, the current to recharge the AC compressor battery is still needed.

 

Maybe assess the battery first. Dr Prius app (with a bluetooth OBD adapter, Carista works well) will give you some idea where it's act. Even just the static test. (see attachment, demo mode)

^ The pipe being this (highlighted) bit? If so, I would do the rest, including EGR valve/cooler, and intake manifold, ASAP. See top two links in my signature. (on a phone turn it landscape to see signatures)

 

depends on how long you want to keep it. if you like it and are not concerned about a blown engine or bad brake actuator, get a new battery.
if not, look into reconditioning or salvage or a rebuilder with a decent rep and warranty.