Belisarius - Looks like they deleted your last response because it is in my mailbox, but not here.
Your gf doesn't have a 5 issue, but my co-worker does...directly after the upgrade. We're both small test cases. The people in your town don't have the issue that you know of. About 50% of my friends/co-workers do (various phone models.)
Didn't you post earlier about race circuits and battery imprints and so on? If the new OS is using a different imprint (amperage or however you phrased it) and that breaks existing batteries that were working fine...then that sounds to me like an Apple issue. Yes, replacing the battery fixes it, but Apple caused it.
Going back to the laptop example... After 20+ years in IT, I've never heard of a windows upgrade causing a laptop battery to go bad or have bad performance. Also we can accurately test laptop batteries. Heck there are utilities from the manufacturer that will tell us when the battery needs replacing.
In my case and many others,Apple is testing and saying the battery is good. If the battery is truly bad, then where is their accountability. Why can't they show us that? I'd be a whole lot happier if the battery did test bad and then they told me I needed a replacement. It would still be a hard pill to swallow because it started after the upgrade, but at least there would be some evidence to back that up.
When a laptop battery is bad, it takes the full time to charge. However, it discharges quickly. It doesn't jump around. If you plug a discharged laptop battery in, it fills up slowly. It doesn't jump to 30 or 50%. With 10, my battery had the shutoff issue. With 10.2 the level jumped around, but it didn't just shut off. Once it jumped from 95% to 15%, but continued to operate way longer than it should have at 15%. This sounds like the OS improperly reporting what the suggested battery life is.
I'm 100% positive that if I could downgrade my phone that battery would have been fine. I'd also bet that taking that battery out of my phone with 10.2 and putting into another 6 with ios 9 would result in it operating normally. Unfortunately, I have no ability to do these tests. I sure hope Apple is.
As far a whether my non OEM battery was reset when installed, I don't know. I wasn't behind the counter while they worked on it.