An A350-800 is a new derivative with significant development costs, but a 787-8LR is a "slight rework of the 787-8"?
I don't get that.
Both are a straight shrink of the model above.
Where do you see a difference?
The only difference for me is that Airbus will already have done a lot of the A350-800 work. They came close to building this, and had orders.
I'm not aware of Boeing even looking at a 787-8LR
The 787-8 is artificially restricted on paper. Its fuel capacity is the same as the -9's. Boeing does not need extensive modification to bring the -8 to the mtow of the -9. Doing that would give the -8 about 700 miles more range than the -9. SYD-NYC should be possible. It is not enough to do SYD-LHR nonstop but the gap to close is not very big - some incremental weight and a center fuel tank.