These days with computer modelling they have a. Dry good idea of what the performance will be.
They also do have other A350 flying and the 778 is a derivative of the 77L/77W so I'd say they have a very good idea what the performance will be. Every 1kg lighter means less weight in the structure to support it which makes it lighter again. A virtuous circle. The engines alone in the 778 are going to be less powerful and much lighter than on the 77L yet it is expected to reach cruise in a similar fashion.
It's all about compromise, don't forget that the X versions of the 777 will have a newer more efficient, larger, and higher aspect ratio wing, throw in less fuel burn per KG and you arrive at that compromise and its why it will closely match the 200LR in terms of mission capability.
The 350ULR is a smaller aircraft with less ability to carry payload over a similar mission, it will however suit plenty of operators looking for that capability but with less ability to fill it.