as said above, as your intellect increases your need for spirit decreases (kind of ironic), and this happens 3 fold.
1) you have more mana.
2) you gain more mana from things based on your mana such as rapture, sfiend, HoH and replenishment.
3) the spirit you do have is giving you more mp5 than if you had less intellect.
I've been reforging out of spirit since I was in 340 gear and sit on 2k spirit maximum in firelands raiding, but I really don't see top Disc priests going as low as 1.1k spirit...
For me, 2k has been a comfortable level of spirit so far and with 132k mana raid buffed now, it's getting even easier and will continue to do so. I could perhaps see myself sitting on 1.8k with a 378+ gearset at the lowest but certainly not 1.1k
As far as top disc priests reforging out of spirit, I'm going to assume this was during the first week of firelands or before.
In full t11 heroic gear those normal fights in firelands go down so fast you probably wouldn't go oom even if you had 0 spirit.
The t11 encounters were also overgeared and after as much practise at them as these top guilds have it's unlikely their healers had to 'cover' for many mistakes made by the raid (they would rather gear their healers for more throughput and take less healers, bring more dps = shorter fight)