As an update:
I shot yesterday from 4 pm until 9:45 pm, or just under 5 hours.
Two X-T2's, both with battery grips, with all original Fuji batteries freshly and fully charged.
Boost mode on. No chimping. Turning off camera when not in use (about 60% or more of that time period?).
Camera 1: 516 shots and one battery remaining at 33%
Camera 2: 389 shots and one battery remaining at 66%
Total frames for those six batteries was 905 frames, with 33% and 66% remaining respectively.
Average number of shots per battery: 192
If I had run the remaining batteries down to zero my frame count would have been 580 and 517, or 1,097 total for the two bodies.
Battery drain between the two cameras seemed pretty consistent.
I had an additional six batteries available on this shoot, leaving me with an additional four to five hours of "working time" if I needed it.
Again, this wasn't non stop use.
This was utilizing on again off again power saving efforts.
My 12 batteries can be relied upon for only 7-10 hours of work for roughly 2,200 frames with no chimping and turning off the camera as often as possible.
Oh, BTW, it takes a long time to recharge 12 batteries!
My results yesterday were very consistent with my original post...
So there 'ya go.
IMHO: I don't believe someone working extended assignments in the field with limited time/availability to constantly recharge batteries should switch from a DSLR to a mirrorless camera.
This system is simply not up to the task of 12-15 hour days in the field.
Don't shoot the messenger.