Couldn't find a topic with this information so figured I could just make one. So rather than each piece giving you xp bonus, its now set based.
(2) Set: Rested experience consumed is reduced by 30%.
(4) Set: Increases your out-of-combat regeneration in the outdoors, normal dungeons, and battlegrounds.
(5) Set: Gaining a level triggers Burst of Knowledge, dealing (X) Holy damage to nearby enemies and granting you 40% (Primary Stat) for 2 min. Defeating additional enemies extends this effect, up to 2 min.
(6) Set: Rested experience consumed is reduced up to 30%.
With no xp bonus attached to them, I personally see no reason why they have to wait until 9.2 or 3 for heirlooms to work to 60 as long as you level a character to 60 without them first.