Honestly, the end bosses should be overtuned during the first couple of weeks. Forces players to spend a couple of weeks gearing with loot from the earlier mythic bosses, making heroic splits less worthwhile and making the race last more than just a couple of days. By the time "regular" guilds reach the mythic end bosses, they will have even more weeks of early mythic gear, vault gear, spark crafted gear, etc so they won't even feel the overtuning.
This smells like Blizzard realising that the raid will be defeated before the weekend is over unless they did these buffs!