I ran this setup in cata as well leveling in mop, and so have some others (warrior + atonement x4). If you have the patience its works and the mind sear x 4 is pretty silly. But once geared still doing 30% of the damage other classes do is a problem.

It was so depressing to watch someone unload 2 million damage in a few spells, dropping old content bosses before their emote is over with, etc (jstmage!)

I still run an atonement priest (or rather glyphed for optimizing it) as my healer; on trash and tank and spank bosses it works mindlessly easy.