Having 4pc of 391 i won't switch it till i get 2pc 397, the 2pc T13 it's not that good to switch H4pc T12 with LFR pieces, if you have 378 then you should go with whatever piece of T13 you can get your hands on.
switched from heroic t12 4set to heroic t12 2 set plus raid finder t13 2 set
being able to SWD at will is awesome, on average, boss fights only, SWD is now doing 8% of my damage
I don't regret it and advice any spriest working on progression to do this if possible.
Me and our other spriest now stick to the boss during H Yor'sahj and just turn and SW: D the slimes when they are in range. We're are barely off from a kill (4% wipe), and its all coming down to straight damage into the boss (less mind sear, more single target). Taking less damage to help the healers is absolutely needed, and on top of the extra damage to slimes at sub 25%, makes this fight perfect for our 2pc.
I know switching to 4pc is a dps loss, but the added burst from it onto the slimes on the handful of attempts I used it was useful, but I did switch back to double 2pc because I needed the raw single target damage. I like the mechanics of the 4pc, but the amount of orbs you waste is insane, and really wish blizzard would rethink it, or at least give us more abilities that use orbs (empowered VT maybe? or Emp. SW: D?).