Rolling Disc At 59 For A BreakFollow

I am 55 undead shadow priest now. I have been always. I understand holy is for healing instances, and disc is for surviving and healing BG's. I want to just take a break for maybe 1 day out of the 1-70 and go disc, as I intend to at 70 for PVP and PVE.

Now with outlands greens, would I be able to survive long if I went the right spec disc at 59? Has anyone ever gone disc before 70 and still had the same type of Power?

Yes, you'll do great. A Blessed Resilience build, combining 33 points in Holy with 17 in Discipline for Improved Mana Burn might do better than a deep Discipline build at your level. You don't have Mass Dispel, so cutting it to .5 cast time isn't an issue, and you don't have resilience, so Blessed Resilience performs nearly as well as Focused Will. Pain Suppression, etc. would be great, but you'd have to give up either Spell Warding or Divine Fury to get it, and without Binding Heal or Prayer of Mending you'll have to rely on Greater Heal much more than most arena priests.

If you're careful with your XP, you can scoop up some rather good quest blues from Hellfire Peninsula to use in that bracket. The trick is to do only the prerequisite quest chains, and to have friends who are not partied with you clear out all the mobs that you don't have to kill to finish the quests but stand in the way of completing it. The Illidari trinkets are particularly good.