Wouldn't legendaries rolled at level 1 get outscaled pretty fast though? At some point, massive gains in toughness/sheetdamage should outdo most legendary effects, no? And that's assuming you even get legendaries at all, let alone actually useful legendaries.
I can't see this being worth it, assuming 4man leveling at tryhard pace.
Edit: or well, I guess you can pre-learn the Challenge Rift since the reset is on Monday, and you kind of want to do it anyway for Bounty mats, and Blood Shards are super available anyway... Starting to make sense.