In general it is more beneficial when you run 4-man party. The whole process is then faster. That is also the main reason why I have included XP pool timing in the GR Calculator.
Completely agree. I usually skip XP pools as well. It is always a gamble if you find some and if you run quick rifts you will always have more gems upgraded and chance of better items. On high level - yes (i would say above 1300).
I've created an excel file calculator for best GR level to chose in order to get most XP/hour ratio. Check it out and let me know what do you think. Comments are always welcome!
Hope it helps!