Any questions about what scales best should probably hold off for a week or two. Scaling is obviously VERY different as a result of this latest update, and there is no guarantee that they won't "fix" it again after they fix the login issues.
It's almost guaranteed that someone is going to point out to Blizzard that iLevel <20 items are scaling better than iLevel >500 items, and they're bound to set a scaling limit or something in the next patch.
Just sit tight, and marvel at the wonders of expansion release bugs, but don't make any long-term gearing decisions for a while.
And don't get rid of ANY quest rewards if you're levelling... just in case.