Ah I drink too much coffee, now on the drink lots of water come down! =)
I'd like to point you that it is: Stored Value * Mastery * 10% from new passive
So we need far less them you would expect to heal for huge amounts. And Remember that the cap value is your HPS so around ~450k right now.
And about the amount of time to stack this amount of overhealing, in 5.2 a 500 ilvl druid gemmed to Int (Since we will have enough spirit by now, and the -9% cost to reju will help too, and by the time we replace the Set bonus we will already have way more spirit to deal with it), Reju is hitting really high.
If you pre-Hot 7 targets, you will get 450k in ~10 sec.
450*1.2*1.1=594k of healing
594/10 + 30*6/10 = 77,4k per target
This without procs and raid buffs. A moderate guess.
In fact numbers will reach over 80k per target in 10-M raids, with 10sec of Reju Pre-hotting.