Got the iPhone X on Friday and did the 11.1 update, and transferred my watch pairing from my 7+ in the evening before bed, when I charged it overnight as usual. It seemed fine.
Sunday morning I did the Watch OS 4.1 update, which took 3 tries due to an error verifying the update. After that I was down to 4% after 15 hours by bedtime, when I'm normally at 70% after 15-16 hours each day (see the AW3 thread where I documented a consistent 70% left at the end of every day).
I forgot to take the watch off that night for the first time when I went to bed last night, and it was in low-power mode this AM. I rebooted the watch at 8AM and placed it on the charger, with it up to 100% by approx 10AM. And now about 12 hours later I have 75% remaining on the battery.
It seems almost back to normal, except I only had about 3.3 hours of use instead of my usual 5-6 hours. I think it's Watch OS 4.1 not the iPhone X, because the watch was fine on the previous Watch OS on Saturday with the iPhone X. But the Reboot from low-power mode is the only reason I can see why it improved.