Clearly, the A350-1000 has slightly lower volume for fuel in the wing and most probably in the center box too. There's absolutely no way A450-1000 can have higher fuel volume than A350-900. No way.
The -1000 was made after the -900. That always
means refinements to the earlier systems are a possibility. Some examples being
- Slightly better placement of the scavenge pumps can see unuseable fuel drop
- Changes to the fuel gallery can lower the amount of volume used for internal plumbing
- Changes to the inerting system can lower the amount of volume used for internal plumbing
- Changes to main pumps or to engine feed can reduce number of pumps needed, which obviously frees up internal volume
- Refinement of the rotor burst zone can mean relaxing of the size of the dry bay(s).
It definitely is not "clear" and there is definitely "absolutely no way" that the -1000 can have a higher fuel volume than -900. I'd say its unlikely, but very far from impossible.
But it doesn't really matter. Either way, the difference between the -900 and -1000 is background noise when compared to the additional uplift required for this mission profile.
The only way to get higher volume capacity than A350-900 on the A350-1000 is to add aux tanks, thus extra plumbing, additional fuel vent, fuel pump, wiring, fuel quantity probes, structural reinforcement, tank puncture protection (Kevlar lining?), Changes in the flight management system (fuel quantity, CH vector etc.) refueling additional panel.
All those things will have to be designed then built by suppliers. One year certification? It is not going to happen. It will take at least 24 months.
Depends if the original fuel system was ever designed with ACTs in mind. If it was, then integration of ACTs becomes a much simpler process. If not, then the system does need a bit of work alright.
Even if it were to take 24 months - do you know when Airbus started looking at it? Qantas launched Project Sunrise in Aug 2017. That is 24 months ago. So unless you know for sure that Airbus haven't started the ball rolling till this thread appeared on anet and someone in Toulouse thought "oh, that sounds like a good idea, have we considered that yet fellas?"....