If you wanna do a tremendous amount of farming on timeless isle, + some pvp gear, about 530 is feasible. 535 would be about the max assuming you were able to upgrade every single slot with a burden of eternity.

If you run LFR and use burden gear, expect mid 530s. Boxing can be tricky to pull off in LFR on some of the bosses and what type of toons you take and how man, and your group may kick you just for multiboxing even if you're top dps, etc. However I've been able to complete all of wing 1 with 9x frost DK, and all of wing 2 with 6x frost DK. I threw in some flex runs here and there for certain items and my toons are all in the 535-545 range. The toons over 540 ran some flex and got lucky.

With upgrades, you can get craftable 530 helm and boots, and craftable 561 belt and legs.

I'd think with timeless isle, craftable gear and celestials loot you could probably hit 530 pretty easily.

Tricky part is getting near the 550 area - flex, lucky drops off celestials, legendary cloak, and gear off ordos would push you there.