OK, so after a full day of use, here's what I found with 8.1.6.0:
- Battery life is as good as in 8.1.3.0 (which, in my case, is rather good - I consistently get close to 5 hours SOT, normal screen brightness, and no special measures to preserve battery; right now I have 4h 5m SOT, with 18% battery left).
- GPS performance is just like in 8.1.3.0 (that is, not perfect, but passable - lock on time about 15 secs, so-so accuracy), but definitely better than 8.1.1.0.
- Performance wise, the UI feels snappy. Can't really say for sure that it's better than 8.1.3.0, but certainly feels so. The Mi 5 has never been a sluggish phone, but with this update it feels really smooth.
- Solid WiFi and 3G/LTE performance.
- At long last,
a weird bug I found on 8.1.1.0 is working OK now.
All in all, a good upgrade, even though it's not a huge change from the former one.