Sensible post, but you didn't mention the 777-8, which we are also committed to. Seems very close in size to the 350-1000, so I wouldn't understand getting both. Perhaps a case of see which one shapes up best and cancel/convert the other?
Size wise you are correct; the -8 is a tad smaller than the present day -300LR and around 4 meters shorter than the A350-1000. But, essentially, the -8 is a -200LR on steroids, and there's not much point in EK having a large fleet of aircraft able to circumnavigate half the earth. Look at it this way: The A350-900 is the A340/B772 replacement, the -1000 will replace the -300LR, albeit with a bit less range, and the B777-9 will fill the gap between the -1000 and the A380. Add to that a small fleet of 777-8 for the tiny handful of routes demanding 18 hours endurance, if indeed EK doesn't decide to convert those orders to the -9.
Having the -8 as the main replacement for the -300LR would be overkill for the vast majority of routes it will be serving. Reliable numbers are, obviously, not out yet, but the -8 will likely be a good 10-15 tons heavier than the -1000.