Airbus is to postpone entry into service of the A350-1000 by two years, to mid-2017, in order to provide a thrust increase to 97,000lb (431kN) from the previous level of 93,000lb.
It is also delaying introduction of the A350-800 by a similar timeframe, pushing its entry back until mid-2016.
Airbus has been under pressure to bring the A350-1000's performance more into line with that of the Boeing 777-300ER.
With the additional power the -1000 will gain 400nm range with 350 passengers or another 4.5t in payload. The A350-1000's maximum take-off weight will increase from 298t to 308t, and its list price will rise by $9 million as a result of the changes.
Rolls-Royce is also to become the exclusive engine supplier on the -1000.
Airbus is retaining the same fuselage size on the twinjet: a five-frame stretch aft of the wing and a six-frame stretch forward, compared with the A350-900.
The aircraft will have an "optimised, not new" wing, said Airbus chief operating officer Fabrice Bregier, speaking at an EADS seminar ahead of the Paris air show.
It will retain "close to 100% hardware commonality" with the A350-900, he added. The A350-900, the first of the three-member family to be developed, will make its first flight at the end of 2012.
Rescheduling the development of the two other models will allow planning and resources for the A350-1000 to be sequenced around the development of the re-engined A320neo, which is due to become available in 2015, when the -1000 was also originally planned to arrive.
The postponement will allow technology insertion and earlier improvements to the -1000 based on A350-900 test results.
Airbus chief operating officer for customers John Leahy said the -1000 would be able to compete "very nicely" against the Boeing 777-300ER. Despite the -1000's having 15 fewer seats than the 365-seat 777-300ER, Leahy said it would offer 600nm additional range and burn 25% less fuel on a 4,000nm journey. The -1000 was "now in a class of its own", he added.
Rolls-Royce said the higher-thrust Trent XWB engine to power the -1000 will incorporate benefits derived from the manufacturer's Advance 3 technology demonstrator programmes.
Civil aerospace division president Mark King stated that the company knew from early testing of the powerplant that it could offer an improved version. The XWB engine was beating targets for internal aerodynamic efficiency, specific fuel consumption and generating cooler turbine temperatures.
"Having seen demand from customers, having seen what's happened on test bed, we [thought we had] other options," he said. "When we saw what this aircraft could do with an engine like that, we had no hesitation."
King said the manufacturer had the capability to build a larger core to increase thrust to 97,000lb with the same fan diameter and without an impact on fuel burn.
There are six XWB engines undergoing tests, he said, and added: "The compressor is by far the best compressor the world has ever seen."
King said that while Rolls-Royce had secured exclusivity on the -1000, the company had "never sought" a similar arrangement on the -800 and -900 - although it remains the only engine supplier for all three variants of the A350.
"We set out from the beginning to demonstrate, by our actions, that there's no need for another engine," King said. Leahy added: "The market isn't pushing for a second engine right now."
Flight-testing of the Trent XWB, on an Airbus A380, is to take place this year. Final assembly of the first member of the A350 family, the -900, is set to start at the end of 2011 with the maiden flight having been set back to the end of 2012.
Bregier is optimistic over progress on the programme. He said Airbus had managed to "get through all the traps" of manufacturing the large composite parts which are crucial to its fuselage and wing structure.
Qatar Airways is the launch customer for all three variants of the A350.