So I have pretty much scrub setpieces (3/5 furious 2/5 relent) 'cause I started s7 the last week of it and didn't got shit, so now I'm stuck with this scrub gear and I feel it's pulling me back a bit, SO:
I can either wait a week (or possibly a bit more) and get a wrathfull setpiece for +57stam / +27int / +34spi / +59sp / +34resill, or I can buy right now 2 more relent pieces (spending arena points in scrub crap, that's why I'm having second thoughts) for +52stam / +26int / +32spi / +52sp / +32resil
Sure, it might be holding you back a bit, but it's not uncommon to see lesser-geared players outplaying other players in high bracket arena. I remember when I was playing in Season 6, it consisted of me, in half honor/half Season 6 gear, a Ret pally in all honor gear, and a priest in raiding gear. We were able to get to about 2.2k before we hit our peak with our gear levels.
So, basically, it's up to you. I'd personally pick up the Wrathful, because arena points aren't easy to come by like honor, so I would spend it wisely. It's whether you believe you can compete without having the best gear.
Get wrathful. The other pieces will come with time. It's not all about how high you can get week 1. The season will last ~6 months and you'll have time to build up your gear and get to the rating you want. Save your arena points and push to get Weapon/OH/Wand though. These are almost necessary for a priest to get to the desired ~1200 resil to leave gem slots open for more useful stats. Full Furious/Relentless if played well with a good team can easily see you in the 2k+ range.