The new, bigger A350 would use a derivative of the latest Rolls-Royce Trent XWB planned for the A350-1000. One person briefed on the plans said it would boost thrust from the current 97,000 pounds to just over 100,000 pounds.
Airbus believes this would compete well for the majority of airline needs and head off further 777-9 sales.
But some analysts said a key to the project would be how easily Rolls-Royce could expand a full slate of projects as it strives to cut costs, even if leaving the Gulf business to Boeing.
"This sounds like they are making a virtue of its lack of hot-and-high performance compared to 777-9 when maybe it's the case that Rolls can't afford, or doesn't want to, do a much bigger and substantially new engine," said Nick Cunningham of UK-based Agency Partners.
One engine expert estimated the upgrade could cost half a billion dollars and require a bigger fan and new materials.
Rolls-Royce was not immediately available for comment.
To give the new A350 more capacity and compete with the 777-9 on long trips, Airbus would boost the maximum take-off weight to just over 319 tonnes, compared with 308 tonnes on the Airbus A350-1000, the person briefed on the plans said.
However it would sacrifice some range compared with the 8,000-mile A350-1000.
Airbus says it has not made a final decision on whether to launch a new A350 and will provide an update at the Farnborough Airshow in July.
Meanwhile, it has been weighing up what to call the new member off the A350 XWB family, reflecting deeper decisions on market positioning that can affect billions of dollars in sales.
It needs to strike a balance between protecting sales of the A350-1000, by emphasising differences without weakening its long-held mantra of commonality between related aircraft.
Until now, the possible new model was widely known in the industry as the A350-1100, continuing a sequence from the 276-seat A350-800 to the 315-seat A350-900 and 366-seat A350-1000.
Now, sources say it is being pre-marketed with a surprise new identity, the A350-8000, though a final decision has yet to be taken. An earlier working title was A350-1000 XL.
Leahy confirmed that Airbus was reluctant to ratify the industry's nickname of A350-1100 but declined to give details.
"You don't want it so close to the 1000 that it is an either-or decision. You have the 1000 and another airplane, with equal gaps of 40 seats between the 900 and 1000 (models), and then whatever this becomes."
Eight is a number widely used by planemakers and is seen as a symbol of success in a key battleground for sales: Asia.
"Eight is a very nice number out in Asia, but we are not going to comment until we launch the programme," Leahy said.
Airbus will however be hoping not to repeat the omen of the previous model to carry the -8000 model number: a VIP version of its A340, only one of which was ever built.