I can confirm now that ilvl 515 req 70 gear drops for
level 75 characters. It can also be obtained from
Bismuth Lockboxes opened by level 75s. However, do note that depending on the source, the lockboxes typically attach a level requirement to themselves when looted, and this matches that of the character looting them. I'm not sure about regular drops, but any boxes pick-pocketed by level 80 rogues require level 80 to open as well.
At the same time, boxes looted from the
Coral-Fused Clam (and possibly any other similar treasure) don't get this treatment, and require only 70 to open, even when the looting was done by a 80. We've opened two such boxes to test, one with a level 70, and one with a 75 (the boxes themselves were both looted by 80s). The first box dropped an ilvl408 piece, the second an ilvl 515 one. This suggests that the loot is not actually generated when the box is, blizz just dirty-fixed the lockbox sources themselves to strap on a level requirement for opening the box as a way to prevent them from being funneled to lower levels. There may be a way to bypass this, but I don't currently know one.
In any case, outside of grouping and dual-boxing, the best way to farm the 515 gear right now seems to be making a 75 rogue and pick-pocketing everything. Unlike the group-with-an-xp-locked-char-and-farm-whatever, this method carries no inherent time limit (as long as you vanish instead of fighting if spotted), and has no second person/account restriction. It also comes with the convenience of being able to pick the locks on the boxes on the spot. Happy farming (just try to leave some for us, if possible)!