Have you considered doing a factory restore and set it up as a new device? I noticed my battery draining on my 6 Plus and couldn't figure out why but factory restore and setting up as a new device did fix the battery drain issue for me and I can easily get over 10+ hours on my 6 Plus.
If battery life is that a concern for you and you feel like you should be expecting more life then you should tweak your settings, like turning off the features that you don't use. Or simply try doing a factory restore and set up as a new device and see if that helps.
I would not be concerned with what ibackupbot reports because ifixit reports that the battery capacity for the 6 Plus is 2915 MaH. And again, battery does fluctuate so give it some time.
Now if it really bothers you because it's reporting that you are getting 2900 MaH, and you are within 14 days, I would just simply return it and get a new one.
I got a 6 Plus Verizon model yesterday and here it is:
Battery
CycleCount: 0
DesignCapacity: 2855
FullChargeCapacity: 3100
Of course it doesn't mean much but once you use it, battery will fluctuate.
