JP gives 378 this patch, so no issue there.
That is correct but in order to jump straight into new heroics while wearing 353/359/365 gear, you need to save justice points. Blizzard should not assume that everyone is hoarding JPs just to purchase 378 gear to pump their ilvl for new dungeons. People who were running 5mans and are fully geared from said 5mans should be able to queue right away.
My average ilevel is 360. I have 365 crafted weapon, all 365 pieces available from MF dailies/Thrall quest (cloak, neck, 2xring, belt, trinket, relic), six 359 items (googles, T11 chest, T11 legs, T11 gloves, boots, darkmoon trinket), one 353 item (bracers from ZG) and one 346 blue item (shoulders, i was unlucky and didnt get any from ZA). With my 4000 justice points saved, I will be able to buy T12 legs and T12 gloves (the biggest upgrades), which will bring my average ilvl to 362, which may still be too low.