How exactly are you going to level your resto druid?

[nocendi]

This hampers your leveling greatly (not being able to use your artifact weapon until you get your 2nd at 103).

The absolute best, most efficient and fastest way is to grab a few friends (with a caveat). Grab your friends and NOT be in group. This way you get the full xp gained as long as you tag the mobs with moonfire/sunfire. This has been tried and tested

You can use the dps artifact weapon from 100, and just stockpile the artifact power tokens until 102.

[ItachiZaku]

However unless you never plan on using an offspec until you maxed out your resto artifact, which could very well be a few months in, and unless you always plan to do quests and stuff with friends; there is no real downside in putting up to 13 points into your offspec weapon.

I figure, if it starts getting too slow or boring, I can switch and pump points into the offspec, but it isn't my initial plan.