I used fully upgraded and enchanted heirlooms; missing only the fishing ring. Leveling from 1-20 took ~3 hours, at which point I immediately turned on War Mode for the duration. I rarely encountered enemy players, and when I did they almost always left me alone. But the few times I did get ganked, the total cost in time was insignificant. Maybe a total of 5 deaths from ganks in the entire 40 hours.
While the expansion kicks off with heavy references and scenes of this war, leveling through the Horde side of the new content it barely came up again. Except for all the times I had to mine Azerite to make sure my faction was the leader in the Azeroth version of a nuclear arms race. The Alliance side of things had some heavier references to to the war, at least in the portions I played, with one scenario in Stormsong Valley where a fleet of Horde aggressors bombed a town.
Fourth: When the enemy faction controls the Warfront, you get access to 11 quests that allows you to trade materials for AP and honorbound rep. This is huge because you get 750 AP per turn in and 500 rep per turn in which gives you a total of 8250 AP and 5500 rep; that is a massive amount. All the material you got from scrapping all your leveling gear will come in handy here and I imagine that you will be able to trade other players for different turn ins. I highly recommend trying your best to do all 11. Also, this will be a great opportunity to make some serious cash for your profession because people will be buying the same items in bulk.
Some people have made arguments that quest blues can sometimes be more powerful than heirlooms, especially with sockets. I'm not convinced. The simple fact that you never have to even look at quest rewards will save a non-insignificant amount of time over the course of leveling. Also of consideration is the value of enchanted gear never having to be re-enchanted.
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