Lots of players are in the habit of getting pre-raid Best-in-Slot gear before stepping foot into end game content, and while that may be a requirement for some guilds, it really isn't necessary in Season of Discovery. These raids are not designed to be gear checks, and out-gearing them significantly will just lead to burnout earlier.
If the transition from Phase 1 to Phase 2 is anything to go by, having a full set of Phase 2 Best-in-Slot gear will likely be overkill for early Phase 3 raiding. While the new items absolutely are upgrades, the overall powerlevel when fully geared will likely not be as insane as the difference between say a Molten Core raider and an Ahn'Qiraj raider.
A Mage upgrading their Electromagnetic Hyperflux Reactivator into Visage of the Exiled will gain 5 Intellect, 3 Spirit, 5 Spell Power and 1% Hit. Obviously a noticeable difference, but not make or break. After seeing the reitemized Dungeon items, they are mostly on-par with Gnomeregan items for most slots. Supplemented with some quest items found while leveling, or dungeon items for those leveling that way, you're unlikely to need to grind any gear before stepping foot into Phase 3 raiding.
While you absolutely could put in the effort to make sure you get your upgraded Blood Moon pieces before you step foot in the overhauled Sunken Temple for the first time, do you really need to?
Looking at the datamined items leaves us with a lot of items that are barely an upgrade over their original versions. The difference between Band of Boiling Blood from Phase 3 and Emberblood Seal from Phase 2 is a whopping 1 Fire Spell Damage, at the cost of 2 Intellect. On the other hand, the difference between weapons could be more significant. Upgrading from Sanguine Crusher to Parasomnia is more than 10 DPS. Even when compared to Thermaplugg's Rocket Cleaver, the DPS gain isn't insignificant.
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