There are 2 x 6+ and a 5s in my family with 8.4, all paired to their own Apple watches. One of the 6+ is jailbroken, and none of them have such problem.
The jailbroken 6+ only installed F.lux, iFiles, CCSettings, and remove the shutter sound from the system file.
So far, I didn't face any problem that may be jailbreak related (both on the phone and the watch). May be I didn't dig dip enough, but so far everything ops normal.
In terms of battery life. I will say more or less the same as 8.3, enough for one day use (10hr+ usage time).