Some items in Legion will not have primary stats (Strength, Agility, Intellect), and last time I checked those items were necks and rings (maybe trinkets too?). To prepare players for this, the Demon Invasion gear reflects this change.
Invasion gear is great for gearing up alts so they will have easier leveling in Legion. My profession alt had around 620 ilvl, now he have around 690 thx to invasion. The gear is not meant to be used for competitive raiding.
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Less stats in total doesn't matter at all. Everyone is in the same boat, and the game is balanced around that. The fact that they have only secondary stats also make them less of a priority over items like relics and trinkets, which makes it a significantly lesser deal if you don't get the best one right away over some guild mates. A 850 amulet only has about 200 less secondary stats in total than an 875 one. That's not going to be a make-or-break factor for most people.
You should be able to get almost all 700+ gear from invasions and upgrade your weapon from invasions to 725.
Read some guides on your class/spec. Optimizing your talents and ability priorities should help a lot. Replacing those cata pieces are going to help too.
Not all gear has the best secondary stats for your class/spec; example: there are no invasion weapons for a havoc dh with crit.
You also have the option of farming baleful gear from tanaan and using the upgrade tokens when you get a piece that has the right stats; if really lucky, you can end up with 705 pieces with perfect stats after the 2/2 valor upgrades. Baleful drops a lot from rares, the upgrade tokens sometimes, or you can buy tokens for 5k and the upgrade token for 20k apexi (weapon 10k).
Do Kazzak and use bonus roll token on him, for a chance at >700 gear.
Overall, don't worry about numbers that much, worry about doing the best at perfecting your play at your class and the numbers will improve both from your gameplay and from gear upgrades.
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