I have both the 5X and the 6P....
Battery wise, the 5X isn't that far behind the 6P... The smaller, lower resolution screen evens the score a bit. It's only when you start ripping on some stuff... Like tethering, for instance... That doesn't care what screen size you have, that's all battery and that ~700mah difference shows.
I can get well into the 3 to 4 hour range over the course of a full 24 hour charge cycle, which is more than enough for me.
I was honestly pleased with the battery performance... A little surprised it did as well as it did. It does as good, if not a bit better, than my wife's S7.