OK here goes, ignoring a whole bunch of variables, just looking at the "steady state" and using some slightly suspect numbers...
distance = 913Kms = 1 hour 787 = 1.06 hours AB
fuelburn/hour 787 = 9000, AB = 10000
so for 913kms burn 787 = 9000, AB =10,617
seat 787 = 219, AB = 250
assume fuelcost 2.05/us gallon = $0.308/lb (approx)
Delta fuelcost 787-AB = 1,617lbs = $498 (AB more)
Assume average saleable $250/pax then revenue 787 = $54750 AB = $62500 and Delta earnings = $7750 (AB more)
"incremental" is added $498 cost but $7252 (nett) revenue
ratio incremental revenue/fuelcost (hours basis 787 distance)
1 hour = 14.5
2 hours = 6.8
3 hours = 4.2
4 hours = 2.9
5 housr = 2.1
In other words on basis up to 3 hours (787 distnce) there is some advantage in terms of revenue potential to operating the AB. with average revenue $250/pax.
(scuse spelling etc, it's this dratted keyboard)
p.s. I'm an accountant