Xiaomi supports Bluetooth LE (4.0 +), but no ant+. The ant+ service is visible, I try to download ant+ plugin and radio service, but I'm not able to make it work.
If you have a Bluetooth HRM, try with this protocol, it should work.
For what I read about the battery life, miui 7.5 is the best rom for mi5. Miui 8 is enhanced at every release, battery draining is every time lower, but doesn't reach the level of 7.5.
I have solved the issue of long press for Google now in this rom.
In default apps, I hadn't choose the default voice input app. Selecting Google now, it work like a charm.
My mi5 32gb experiences call dropping when I'm moving with car. It happens when signal changes from h/h+ to e, switching a second to no signal and then to edge, but in this way the call drop. It seems that the phone is slow switching by this 2 bands.
Now I'm on 8.0.12 eu stable but it happened...