Just to share my experience in leveling alt right now. It is totally possible to level an alt from level 100 to level 110 just using Legion invasion alone. I have already leveled a priest this way since 7.2 dropped. And I did not even do the starting artifact quest, which I intend to do after reaching level 110. This expansion has a lot of story which can only be experienced by having all 12 classes, therefore I am leveling all of them. Without the artifact quest to unlock other quests, can still fly over to the main land to grab FP and do the invasion world quests when they pop.

More good news on the way is that according to dev, heirlooms will be upgraded in patch 7.2.5, which will make leveling alt even faster than.