Good post RJMAZ - the only Caveat the more I think about it though is my previous thought that the 8 never sees the light of day and that effort/funds are put into stretching the -9 into an -10 as long as possible - call it 85M.
Although it would hurt it on range - it's economics up to 6,000nm would be phenomenal with up to 100 more seats or 50/65 3 class/2 class.
A stretch actually reduces range more than you think. A stretch to 85m would have a range well below 6000nm.
A stretch means increases fuselage drag requiring extra thrust to reach minimum climb levels. See the 787-10 engine thrusts.
A stretch means extra empty weight so with the same MTOW weight fuel must be reduced.
A stretch means more cabin area so a heavier standard payload and even less weight available for fuel.
The landing weight will be much higher most likely requiring a stronger landing gear even if the MTOW is kept the same. The payload might have to be slashed to keep landing weight the same.
At around 2000kg of empty weight for every metre of 777X fuselage it adds up quick.
My estimates are as follows:
777-9: 76.7m 181T OEW range with 410 passengers 7525nm.
80m stretch: 187T + 4% thrust/fuel burn 4% less fuel carried. Range reduced by 8% to 430 passengers 6925nm
82m stretch: 191T + 6.5% thrust/fuel burn 6.5% less fuel carried. Range reduced 13% to 445 passengers 6575nm
84m stretch: 195T + 9% thrust/fuel burn 9% less fuel carried. Range reduced 18% to 460 passengers 6150nm
85m stretch is below 6000nm and the engines would need 10% more thrust which might be difficult.