So... full report.
As noted, battery life stats are different. This has made it hard for me to do a comparison but subjectively, battery life seems about the same as iOS 11 for both my X and 6+. Not seeing an extra hour or anything like that.
Performance-wise, X is more responsive to gestures and is now on par with my Android phone (finally), which is good. iPhone 6+ still has home button delay that's measurable using a high speed camera. I think Apple doesn't bother because they anticipate their future phones to have no home button. Performance-wise, 6+ is a bit smoother in animations, but app loading is not any faster, likely due to meager amount of RAM. X actually seems to open up the Facebook app quite a bit faster now after a clean install of the OS.
Certain apps are still crashing for me upon launch and on both phones so I guess those are the apps and much less the OS.
Overall, not bad. Surprisingly usable as a daily driver. No major deal breaker yet.
Then again, the notification situation is still horrible. I no longer have to scroll down a lot to see something but clearing all notifications is still quite a chore. iOS 10 with the old notification drawer did this a billion times better than the weird behavior introduced since iOS 11.